Do we know where dark matter is?

Daniel and Jorge talk about where dark matter is in the Universe.

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Episode Transcript

Speaker 1 (00:08):
Hey, or hey, I think I may have lost something.
Oh did you lose your mind again? I happened a
long time ago. This is something new? Is it important?
It's actually kind of a big deal. Is it like
really small and easy to lose, like your keys or
your wedding band. No, it's much more embarrassing because it's
really enormous, like cosmically large, like most of the stuff

(00:31):
in the universe, and you can't find it. I've been
looking everywhere for it. Well, I guess you know what
to do. Oh yeah, what's that? You know? Give it
a cool sounding name, and then ask the federal government
to help you find it. Do you think anybody would
actually fall for that? I think they already have. Hi

(01:03):
am jorhammy cartoonists and the creator of PhD comics. Hi.
I'm Daniel. I'm a particle physicist and a professor you
see Irvine, and I am actually paid to hunt for
missing things. M interesting, You're like the Lost and Found
of the universe department. That sounds very unglamorous. I like
to think of myself more of the Sherlock Holmes of
the universe. I see you're not in like in the

(01:25):
basement office with a little window that people go there
and claim lost things. That's right. I'm not sitting here
surrounded by people's lost purses or socks. Where do all
the socks and the dryers go? That's what I want
to know. We should have a whole episode about that.
They're orbiting the Earth in the hose zone layer. Well,
that socks made. They're all connected by wormholes. You think dryers, Well,
that's spinning and that he could that create a warmhole.

(01:48):
Keep running your dryer let us know. But welcome to
our podcast Daniel and Jorge Explain the Universe, a production
of I Heart Radio, which we operated as a sort
of lost and found of ideas about the universe. Everything
we have found about the universe and all the ideas
we have lost. Everything we do know about what's going
on out there, the size of the universe, the number
of dimensions of space, whether or not they're alien creators

(02:11):
on other planets looking through their telescopes at us. Every
deep and enormous question about the universe you might consider
we talk about here on the podcast. Yeah, because it
is a huge universe full of amazing and wonderful things
that seem to be screaming at us for us to
find them. We get light from distant stars all the
time at all times, radiation, cosmic rays, and it's all

(02:32):
coming to us sort of one thing for us to
find out and what's out there and to discover and
learn about how it all works. You make the universe
sounds sort of cooperative, like it wants to play a
role in our science and be helpful. Universe, right, I mean,
it's constantly shouting at us, isn't it. It is? But
sometimes these clues are very, very subtle and frustratingly difficult

(02:53):
to grasp. I feel like the universe is sort of
playing cat and mouse with us, sometimes hiding all the
best bits. It's like coy universe, mean, like it wants
us to find it, but it's not telling us everything
that it can about itself. Yeah, it doesn't just come
right out and tell us it's deep nature. It sometimes
seems to make sense, and then you dig deeper and
discover oh my gosh, it's bonkers underneath. But there is

(03:15):
a lot of fascinating information in those little hints that
do arrive here on Earth. Yeah, and it's pretty amazing that,
you know, if you think about it, that we're sitting
on this little rock floating through space in a corner
of the galaxy, in a little corner of the universe.
And somehow, from this light that we're getting from distant stars,
we can somehow piece together the whole structure almost of
the universe. I like how you just say somehow, you know,

(03:37):
It's like the information comes and dot dot dot, we
know these things. You just like, somehow my entire career
right there. Yeah, somehow it goes to the lost and
found department in the basement and we get a little
memo about what happened. That's right. Somehow, because the government
decides to fund basic science research, we have uncovered some
truths about the universe. I guess what I mean by

(03:59):
somehow thought dot dot is you know, mostly engineering and
then some science, some happy collaboration between scientists and engineers.
But yeah, we are learning more and more about the universe.
And there's a big question, especially a big question about
the universe, about what it's made out of and how
it's what structured it has out there in the galaxies
and also in between galaxies. What does it all look like,

(04:22):
how's it all put together? It's really one of the
biggest questions in the universe, and one of the first
questions you might ask, which is what is in the universe?
What's the universe made out of? Where is all the
stuff out there? And what kind of stuff is it? Really,
it's sort of shocking and amazing that we don't know
the answer to that very basic question about the nature

(04:44):
of our own reality. Yeah, because the universe sort of
pulled a fast one on us, right Like it did
this big reveal twist somewhere a few decades ago where
we thought we knew what the universe was made out of.
It was stars and matter and atoms and things like that,
But suddenly we learned that that was only like five
percent of it. You know. It's kind of like when
you're watching a show and Sony you realize that you

(05:05):
still have like ninety five episodes ago. It's sort of
like the universe was wearing a mask and then somebody
pulled it off and we discovered, oh my gosh, there's
a lot more to it. So much for the universe
being helpful and revealing, right, well, I think it just
wants you to keep watching, I guess, trying to parse
out the information, trying to keep it interesting. Maybe the
show runners of the Universe are just trying to dole

(05:26):
out the reveals a little bit at a time. Show runners, Huh,
so you're a multitheist. I think it's got to be
a committee. I mean, who, there's so much to do.
That's not a flattering I guess description of it looks
like it was made by a committee. The universe looks
like it had too many writers. Well, it does sometimes
seem inconsistent, you know. Well, it is sort of a

(05:48):
mysterious question. What is the universe made out of? And
as we've learned in the last few decades, it's not
made out of just stars and atoms and elements. It's
mostly made out of other things, namely dark matter and
dark energy. The kind of stuff that makes up you
and me and hamsters and ice cream and bologna sandwiches
is actually quite unusual in the universe. Most of the

(06:11):
stuff that's out there is not stars and gas and
dust and all the visible stuff. It's something else, something dark,
something we've only recently discovered exists out there. Yeah, and
so dark energy is this sort of the phenomenon where
the universe's expanding faster and faster every second, but dark
matter is especially sort of weird and concerning because it's matter,

(06:32):
it's stuff. It's like exerting gravity. You can feel it,
but we can't see it, which means we don't know
where it is exactly. We know that dark matter is
some kind of stuff. There's something out there exerting gravity
on the rest of the universe, and we can sort
of tell that it exists, but we don't know what
it is. And we've done a lot of podcasts digging

(06:53):
into what it might be. Is it axons? Is it
black holes? Is it whimps? Is this something else? Something
weird in there? Even more crazy ideas we haven't yet covered.
It's sort of like we know there's an elephant in
the room or in the in the galaxy or the universe,
but we don't know what it's. What this elephant is doing,
or you know, what is it like striking a post?
Is it jogging? Is it sleeping. It's kind of a

(07:15):
big mystery, like we know it's there, but we sort
of don't know what it's doing or what structure it has. Yeah,
you can ask so many fascinating questions. So far, we've
mostly focused on what is dark matter? Is it this
kind of particle. Is it that kind of particle but
equally interesting without even knowing what it's made out of,
is just wondering, like where is the dark matter? Is

(07:36):
it here with us in this room? Is it out
there in deep space? Does it form planets and other
kinds of structure? Is it smoothly spread out throughout the universe?
Where in the universe is all of this stuff? So
today on the podcast, we'll be asking the question, how
do we know where dark matter is? It sounds like

(07:59):
he lost you lost it again? Then I feel like
we said a new word. It was, but now we
don't know where it is. It is a really important
question because knowing where the dark matter is can help
us get clues about what it might be, because certain
kinds of dark matter might clump up in different ways,
and other kinds of dark matter might not. Yeah, is
it is it chunky or smooth? I guess it's a

(08:20):
big question about the peanut butter dark matter of the universe. Well,
I hope the showrunners don't disagree about that, because then
you might get chunky parts of the universe and creamy
parts of the universe. You're not a creamy peanut butter fan.
It's all about the chunks, man, It's all about the chunks. Well,
to each their own may there's a different flavor of
dark matter for every tape. New Tella there you go,

(08:40):
New Tella is the best flavor of dark rutter and
everyone gets a heart attack from being surrounded by all
this saturated fat. But yeah, it's a big question. Where
is dark matter and sort of like what structure it
has in the universe. Is it sort of smoothed out
there like a big cloud or is it is it chunky?
Is it in like strands? Is it in clumps? What's

(09:03):
it doing out there? And this is something that scientists
are eager to figure out because they just want to
know where all of this stuff is. They're trying to
develop a picture of the universe both visible and invisible,
and of course the invisible stuff much harder to see,
but since there's much more of it than there is
the visible stuff, it's a very important question. So, as usual,

(09:24):
we were wondering how many people out there had thought
about this question of the location and structure of dark matter.
So Daniel went out there to ask people on the internet,
how do we know where dark matter is? So if
you'd like to put your brain in the test and
answer questions that leading physicists don't know the answer to,
please write to me two questions at Daniel and Jorge

(09:45):
dot com. Here's what people had to say. Well, doc
matter reacts with gravity, so by looking out into the universe, Um,
we can sort of detect where the doc matter is
because of its effect on gravity. I know that it's
by gravity. Is the gravitational lensing that makes it possible
to have like some kind of ideas where it could

(10:05):
be dark matter in the universe. The distortion of light
suggests that there are more matter out there that we
can actually say whenever it's like bands. Well, once again,
I assume this has to do with the pot with
the examples and instance, as you were talking of these
massive tubs of argone if I'm not mistaken, that are

(10:28):
deep underground. We cannot see what dark matter itself is.
Light travels straight through it, but we can see other
planets and we can see light being distorted by the
gravitational effects of what dark matter is itself. I'd say
we know where the dark matter is because we can
see the gravitational force it exerts on the visible matter

(10:49):
around it. Next to that, it also bends light from
distant galaxies when it comes towards us. So I'd say
gravity is the usual suspect for us knowing where dark
matter is. Who is d M? I think I've heard
that dark matter is everywhere, that it just kind of

(11:10):
permeates the universe, so all over the place. So my
guess is for what we know dark matter is, or
how we know it is, if it gives off gravitational
waves because it interacts with stuff through gravity, then we
would use something that detects gravitational waves to see either
how far away it is or where it is in

(11:34):
respect to something else. I think we know where dark
matter is by looking for localized gravitational effects like lensing
or relative velocities that aren't completely explained by matter we
can observe in other ways. Right, A pretty interesting answer
is a lot of people went with gravity, which is true, right, Like,

(11:54):
like that's how we initially discovered dark matter is through gravity. Yeah,
that's basically the only way we can sense dark matter,
and so gravity is basically the answered. Gravity is the
reason we know dark matter exists, and dark matter is
basically an explanation for otherwise unexplainable gravity. So yeah, gravity
is basically our portal into the dark universe. And I

(12:16):
like this person who said who is dark matter? It's
like who it this new phone? I think that's because
in the question I wrote d M instead of dark matter,
assuming that everybody would know what DM meant, right, who
doesn't know what DM is? I was sliding into some
of these d M s with that one. I guess

(12:37):
you're being a physicism using acronyms on people who had
no idea what those acronyms are. But yeah, a lot
of people seem to set and have a basic idea
that it's through gravity. But the picture is a little
bit more complicated than that, right, I mean, we sort
of know that it's there because of gravity, but sort
of finding out exactly where it is or whether it
clumps or strands or smooth, that's much harder because we

(13:00):
can't see it right exactly. We can't see it in
the way that we can see the other kinds of matter.
It doesn't give off light, it doesn't reflect light. Remember
that dark matter is a bit of a confusing name
because dark matter is not actually dark. It's transparent, it's invisible.
It's not like a cloud of dark matter between you
and another star would block your view of that star.

(13:21):
If that were tue would be much easier to see
dark batter than it is today. Instead, light passes right
through dark matter. So give us, maybe start us off
with a refresher of dark matter or d M as
the physicist call it. You know, what do we know
about it? Well, we know that dark matter is about
twenty five of the energy budget of the universe. That
means if you take like a cubic light year of space,

(13:42):
or any volume of space, then of the energy in
that space is devoted to the mass of dark matter,
whereas five percent of the energy budget of any chunk
of space on average, you know, averaging over big distances,
is devoted to making things like stars and gala seas
and dust and giraffes and all of that kind of
normal matter made out of atoms. And that means that

(14:05):
there's a huge amount of dark matter. That a galaxy,
for example, is mostly dark matter. That the universe, the
stuff in the universe, the matter, you know, the physical
form of the universe is mostly dark matter. So we've
been studying the universe for thousands of years looking up
at the sky wondering how things work, and only recently
have we discovered that we've been missing most of it.

(14:27):
So that's pretty exciting. And we know that dark matter
is not made out of atoms, not made out of
the kind of stuff that you and I are made
out of. If it were, then it would probably interact
with light. And we can also do some careful accounting
from the very beginning of the universe, where we know
something about how much material there was to make atoms,
we can kind of account for where all of that went.

(14:47):
So we're pretty sure. We're almost certain that dark matter
is some kind of matter that doesn't give off light
or reflect light. It must be made out of something else.
And we know that it doesn't move very fast. We
call it cold dark matter, the as if it did,
it would spread out much more throughout the universe. Yeah,
and it's kind of interesting because I think I almost
feel like like, in a way, physicists called this thing

(15:08):
or named it a little bit too early, you know
what I mean, Like we gave it a name like
a dark matter, maybe a little too early, Like maybe
you should have kept going and say that you know
of the universe is just something that we don't understand
or something that is not like the rest of the
stuff in the universe. Well, that doesn't work as well
in grand proposals as a nice zingy phrase like dark matter.

(15:31):
But yeah, but I know what, I guess the point
is it really we don't know that much about it.
I mean, we sort of know it's presence, or at
least we know it's the effect on the rest of
the universe, but we don't even know if it's matter. Right, Well,
we know that it generates gravity, which suggests that it's
curves space according to general relativity. And so either our
understanding of how space curves is wrong, or it's some

(15:53):
new kind of energy and matter that does curve space.
And it's possible that we don't understand gravity. It's certain
that we don't have complete understanding of gravity. There are
alternative ideas to explain dark matter using variations on gravitational theory,
but none of them can really explain everything that we see.
And so you're right that we're not certain that it's matter.

(16:15):
But it's the simplest explanation that fits all of the data.
A new kind of particle that only interacts gravitationally, explains
basically everything that we see out there in the universe,
from the ripples in the earliest light to the structure
of the universe today to the rotations of galaxies. So
we're not certain, but it's the best candidate. Maybe should
have called it dark probably matter or dark most likely

(16:38):
matter d M M L D d M l M.
But we, as you said, so far, we only know
about it because of its gravitational effects, right, But we
sort of know quite a few things about sort of
generally where it is. We do have a good idea
of where it might be because of its gravitational effects. Right,
it is invisible. It doesn't give off light or interact

(17:00):
with any other kind of force. But gravity is local. Right,
if you are close to something, it tugs on you
more strongly than if you're far from something. So if
you're measuring the gravity of an object, if you can
only tell if something is there because of its gravity,
you can get an idea for where it is based
on what it tugs on. For example, we can see
the black holes are there because of the way the

(17:21):
stars move around them without actually directly seeing black holes,
and so gravity definitely can give you a picture as
to where things are in the universe, and we have
a rough idea for where dark matter is. We think
that dark matter is mostly lined up with the normal matter.
That where you see a galaxy is where there's a
huge clump of dark matter. So every galaxy, we think,

(17:43):
for example, is embedded in a huge cloud. We call
it a dark matter halo for every galaxy. Yeah, it's
like where you see regular stars and planets, you see
dark matter. Or it's almost like the opposite, right, it's
like where you see dark matter is where all the
stars and planets formed. In a way, yes, stars are
more like the tracers for the rest of stuff, right,

(18:04):
dark matter leads the way. There's more dark matter than
everything else. And so it's actually like where the dark
matter started clumping is where the normal matter fell into
it because of its gravity and then formed galaxies and
stars and all kinds of stuff that we can see.
So you know how when the military is fighting at night,
they shoot bullets and then occasionally, like one out of
every thousand bullets is a tracer. It's like glows, so

(18:27):
they can see where they're shooting. Stars are sort of
like that. They follow the dark matter, and they give
us a clue as to where that dark matter is.
And that's why we think that most galaxies are embedded
in this cloud, this halo of sort of spherical, sort
of a little bit elliptical dark matter that goes well
beyond actually where the stars are. Yeah, I've heard this

(18:49):
sort of analogy that regular matter like planets and stars.
It's sort of like the sprinkling on the icing of
a cupcake. Like, not just like in terms of our
relative importance to that and this the eye of the universe,
but also kind of like you know, sprinkles stick to
the icing in a cupcake. You know, you can't sort
of have sprinkles anywhere else. They're like, you know, the
sprinkles sort of tell you where the icing is. Yeah,

(19:12):
the sprinkles are sort of the stars, and the icing
is sort of dark matter, and the cupcake itself is
dark energy. That gives you sort of a sense of
the relative fractions of the energy budget of the universe. Yeah,
so I guess we are just the hangar ons of
the universe. We're just hanging onto dark matter. And so
let's get into a little bit more detail about what
we know about this halo around galaxies and also how

(19:35):
we know where it is, but first let's take a
quick break. All right, we're talking about dark matter and
where exactly it is, because I guess we know it's there,

(19:57):
but physicists can't find it. It's pretty tricky to nail
down an individual piece of dark matter, for example, because
it only interacts gravitationally, imagine trying to find like an
invisible piece of sand in your room. How would you
detect it? It's gravity is essentially nothing because gravity is
a really really weak force. All of the other forces

(20:17):
of electromagnetism, even the weak force, is much stronger than gravity.
So in order to detect something through gravity, you need
to have a huge force. You need to have like
a planet sized force or a solar system sized force
because gravity is so weak. So when we use gravity
to look for dark matter, we can only sort of
tell the large scale structure. It's very difficult to get

(20:39):
a fine grained picture of where things are. But even
though it's very weak gravitation, and we do have a
kind of a pretty good idea of what shape it
has in the galaxy, right like this halo, it's not
just like a blob. It has some sort of shaped
to it. That's right. It tends to be denser at
the core and thinner further out, much like the visible
matter in the galaxy. See, and we can tell where

(21:01):
it is because it has an effect on how the
stars spin. Right, Like, the old familiar story is that
we know that dark matter is there because we see
the speed of stars is way too high. If dark
matter wasn't there, then if the galaxy was spinning this quickly,
you should be throwing its stars out into interstellar space.
It needs more gravity, something out there to hold those

(21:23):
stars in place for the galaxy to spin this fast.
That's the old story that just tells us that dark
matter is there. But we can get much more fine
grained information. We can tell where in the galaxy that
dark matter is by measuring the velocity of stars at
different points as you move closer in or further out
from the center of the galaxy. Right, Because I guess

(21:44):
what you're saying is that if the dark matter was
all clumped together in the very very center of the galaxy,
the stars will move differently than if it was more
spread out throughout the whole galaxy. Right. Yeah, If you
are a star moving through the galaxy, then the thing
that determines your speed is how much stuff there is
closer to the center of the galaxy than you are.

(22:04):
You're not sensitive to anything that's further out than you.
It's sort of like if you dig into the Earth,
then everything that's further out from you, that's above you
doesn't affect your gravity at all because it all cancels out.
It's only stuff that's closer to the center of the
Earth than you are. So it's the same for a star.
The star is speed basically tells you how much stuff
there is between it and the center of the galaxy.

(22:26):
So as you look at the velocity of the star
as you move further out from the center, it gives
you a picture for where that dark matter has to
be to explain that velocity. The dark matter was all
clumped together at the very center of the galaxy, then stars,
you know, halfway out from the disk would be moving
faster because it would be a stronger force from all
of that gravity. Instead, if it's spread out really, really far,

(22:48):
then some of that stuff is outside those stars and
it doesn't affect them. It doesn't pull them towards the center,
it doesn't speed them up as much. Yeah, I guess
it's sort of like, if you're in the middle of
a cloud of dark matter, you're not to feel its
gravitation effects a much because it's pulling you in all directions,
Whereas if you're really really far away from it, the
whole blob, then you are going to feel sort of

(23:08):
its entire gravity. Yeah, and so that's how we know
that it's more dense in the middle. And that's kind
of important, right, It is important, and it's not something
that we really fully understand. Like if you do simulations
and you say we think we understand how galaxies might
have formed, and how this halo formed and all the
dark matter swirls together to make this well that forms
the galaxy, then we predict a certain shape for that

(23:30):
dark matter density. We predicted to be sort of like
peaky near the center, that like most of the dark
matter should be right at the center and then should
fall off kind of quickly. But what we observe when
we go out there and we look at the dark
matter see where it actually is based on these rotation curves,
is that it's not as peaky near the center. It's
more like a broad, flat core, like a big blob

(23:52):
of dark matter, it's not as like pointed at the center.
So that's a current mystery we don't really understand. Dark
matter doesn't seem to be as clumped towards the center
as we thought. And I guess part of it is that,
you know, a lot of people sort of wonder like,
if there is that much dark matter out there, why
doesn't it just collapse into a dark matter black hole?
But we we've talked about before how dark matter basically

(24:13):
is not sticky with itself, like it doesn't feel besides gravity,
It doesn't feel any other force that would make it
stick together, Like our adoms have the electromagnetic force to
make them stick, but dark matter doesn't appear to have
something like that. And that force is important if you're
going to fall into the black hole, because you have
to have some way to lose your angular momentum. Dark matter,

(24:33):
like everything else, is spinning and swirling, and the reason
that things don't fall into a black hole is because
they are swirling around it, the way the Earth is
orbiting the Sun and not falling into it. For the
Earth to fall into the Sun, it would have to
somehow lose its velocity. It would have to bump into
something we have to get slowed down. That only happens
if there's some sort of like sticky force that can

(24:55):
do that. So for a dark matter, that's really hard
because it passes right through itself, it passes right through
normal matter. It's very hard for it to lose its
speed or its angular momentum. So that's why this halo
of dark matter is bigger than the visible galaxy because
dark matter actually finds it harder to collapse, harder to
fall into black holes, right, you know, And so it's

(25:16):
been this big diffusion. It has this interesting shape. So
then how else can we sort of know about its
structure besides it's sort of general blobby shape. Well, one
of the listeners got it right thinking about how dark
matter distorts the path of light. We can tell when
there's a big blob of dark matter between us and
something else because it acts like a lens in the sky.
Remember that dark matter, even though it's invisible and light

(25:39):
can pass through it, it does change the shape of space.
And that means the space can act like a lens,
and so light will pass through it, but it will
get bent on the way. And so if there's a
big blob of dark matter between us and a distant galaxy,
for example, it will change the shape of that galaxy distorted,
just as if there was a lens there. So we
can use that to try to get idea is for

(26:00):
where dark matter might be interesting. So dark matter does
seem to clump within our galaxy? Is that what you're saying? Like,
maybe within our galaxy there are spots where dark matter
seems to be denser than others. That's hard to tell
because this kind of gravitational effect is kind of rare,
Like you need a clear background galaxy and then you
need a blob of dark matter right in front of

(26:22):
it has to be like perfectly lined up, so it's
tough to use this. They call this strong gravitational lensing
to get a clear picture for where the dark matter is,
because we don't have really enough examples, so it's not
a great way to tell where the dark matter is.
A better way to tell if dark matter clumps up
is to look for its effect on stars. So not

(26:43):
just like the velocity of stars as they weave around
the center of the galaxy, but their motion in other directions,
like if there is a big clump of dark matter
and especially dense blob of dark matter. It will affect
how stars are moving around it. It will change the
motion of those stars. It will attract them, it'll reflect them.
I see. So if you look sort of look at

(27:03):
the overall motion of all the stars in the galaxy,
if you see that there are you know, sort of
wiggles here and there or little you know, eddies or
little clumps of stars forming, then you know that there's
something else there and that it's not the dark matter
is not perfectly smooth. Yes, And we recently launched a
satellite called Gaya which is mapping the galaxy in four dimensions.
It measures the position, the location of all these stars

(27:26):
and their velocity. So we're getting this incredible map. It
has like a billion stars with their position and their
velocity map. Then we can use this to look for deviations.
Were like, well, if dark matter was perfectly smooth, what
would we expect all these stars to be doing. And
are any stars doing anything weird? And if they are,
we can sort of back that out and figure out

(27:47):
where dark matter has to be to explain any weird
patterns of the star motion. Right, Because I guess if
dark matter was perfectly smooth, Peanut butter like this kind
of smooth cloud. Then you would expect all the stars
us to be basically moving along as if it was
in a lazy river, right, like everyone sort of moving
at the same You know, nobody would be going much

(28:08):
faster than or slower than any of the other stars. Yeah,
at the same radius, right, we expect as you go out,
as you change your radius relative to the center of
the galaxy, these things will change, just like they do
in our solar system. Pluto's not moving around the Sun
as fast as Jupiter, which is not moving as fast
as Mercury, because as you go further out the gravitational
force is weaker. But you'd expect things at the same

(28:29):
radius to basically be having the same motion. And so
if you see deviations, then you know dark matter is there,
and we do sort of expect there to be clumps.
We expect that the galaxy, for example, has lots of
other galaxies inside of it that it has absorbed. We
think that the history of our galaxy includes lots of
collisions to form the Milky Way, and you would suspect

(28:50):
that those galaxies might still have like their dark matter
halos embedded within hours because I guess you would expect
our matter to be clumpy because regg or matter is clumpy,
so in a way, like wouldn't our regular matter also
sort of catalyze or trigger dark matter to clump. Our
matter is clumpy, But that's because it's sticky, right, It

(29:10):
can form these blobs, so when gravity pulls it together,
it sticks together and then that accumulates forms this runaway effect,
whereas dark matter is not sticky, and so dark matter
halos can pass right through each other without very much distortion.
The other question is a cool one, like do stars
form clumps of dark matter? And this is something people
have studied. They've tried to look to see if there's

(29:32):
like an intense blob of dark matter inside the sun,
for example. But remember that there's much more dark matter
than there is normal matter, and so dark matter sort
of wins the gravitational battles. You would expect it to
mostly go the other direction, that dark matter would influence
the pattern of normal matter rather than vice versa. But yeah,
it is a tug of war. Interesting you're saying dark
matter is ignoring us. Basically it's ghosting us. It mostly can.

(29:56):
But back to this question of like following the stars.
There are some really cool things that we do see
inside our galaxy. We can see the remnants of other
galaxies that the Milky Way has eaten, little mini galaxies
we call these dwarf galaxies, and some of these are
really really interesting because they're super high in dark matter.
Like our galaxy has a lot of dark matter, but

(30:19):
some of these dwarf galaxies are almost entirely dark matter,
and we can tell that they're there because we see
stars orbiting these invisible dark matter halos. So there's sort
of like many clumps of dark matter within our dark
matter halo. Interesting, so it is clumpy, but maybe because
we've we've added the clumps kind of yeah, because we've

(30:39):
formed our big halo from a bunch of clumps. So
we think these clumps formed initially each one of these
its own galaxy, and then galaxies eventually do merge and
collide and form bigger galaxies. And sometimes those dark matter
halos don't necessarily spread out and just join like the
original creamy blob of the Milky Way. They sort of
stay there as chunks, and you can tell of they're

(31:02):
because the stars swirling around those little dwarf galaxies. Well,
here's the question. Do you think the dark matter in
our galaxy is spinning also with the rest of the
stars and galaxies, or is it just standing still. It's
almost certainly spinning. That's the reason that it doesn't collapse
into the black hole in the center. If it was
standing still, then that gravity from that black hole would

(31:22):
just suck it up. So it's almost certain that it's rotating.
That we can't measure that directly, right, we haven't seen that,
but we're fairly certain that it has to be otherwise
it would have collapsed. So through strong gravitational lensing we
can tell that there are some clumps out there, and
through some of these absorbed galaxies we know there are
clumps out there, but what else do we know about

(31:42):
this clumpiness. We can also try to measure the clumpiness
by looking at the effect of our gravity on things
near the galaxy. So sometimes these mini galaxies or these
globular clusters get sucked inside the galaxy. Sometimes they're in
orbit around the galaxy. So for example, the large Magellanic
Cloud is a ab of stuff that's sort of like
a satellite galaxy of the Milky Way, and often these

(32:04):
galaxies get torn apart, they don't hold themselves together. They
turn into these streams. So around the galaxy there are
these things called stellar streams, which are these like lines
of stars moving in a loop sort of around the galaxies.
It's sort of like the galaxy has rings of stars
interesting like accidents almost, yeah, and they're sort of swooping

(32:28):
around the galaxy. And those are very sensitive to the
distribution of dark matter. So if, for example, the dark
matter halo is clumpy, it will affect how those stars
get pulled apart and whether they're like gaps in those streams.
So there are people right now studying these stellar streams.
They're like probes of that dark matter halo to look
to see if there are clumps in the dark matter

(32:49):
halo or to see if it's like perfectly spherical or
kind of elliptical. So these are very nice ways to
tell how much dark matter they're passing through and how
clumpy it is. Interesting, and so that's one way to
sort of know the clumpiness of dark matter. And what
have we learned from all of these different ways. We
don't have a great picture of where dark matter is
in the galaxy. People often write in and ask like

(33:12):
where is the dark matter? Can we see like planets
of dark matter or that's kind of stuff. Really, we're
not very sensitive to the details. We know that the
Milky Way has a big blob of dark matter that
it's probably elliptical, you know, it's not totally spherical. We
can see some clumps where these faint dwarf galaxies were absorbed,
but we don't have a great sense for the structure

(33:33):
of the dark matter. It's mostly smooth, but we can't
see things smaller than like, you know, tend to the
six stars. I see, like the smallest clump we can
sort of tell right now is is tend to the
six billions of kilometers maybe tend to the six solar
masses equivalents of dark matter? Is like the smallest chunk
of things we can tell. Well, that's like our our

(33:55):
best resolution of our picture of dark matter in the galaxy.
It's like a pixel this side a million sons. So
we're not very sensitive and that's just because it's mostly smooth.
There aren't a lot of features to see, who we think,
and because we're not very sensitive to it. Again, gravity
is very weak and it's our only way of interacting
with it. Which makes it kind of frustrating. All right, well,

(34:16):
it sounds like it's still yet to be discovered. Who knows,
Maybe it's forming giant dark matter squirrels or bananas or
grass out there, but we just can't see it with
our current resolution. And so let's get a little bit
into what the overall picture of dark matter then is
in the universe, and also what's happening between galaxies. But
first let's take another quick break. All right, Daniel has

(34:49):
lost his dark matter and his mind apparently. Did you
lose your mind looking for the dark matter? I did.
It's driving me crazy. Where are you? It's avoiding you,
It's else see you. It is a little bit ghosting
us as humanity because we know it's there, but it
doesn't seem to want to make expressence known to us.
In detail, we sort of have a big picture of

(35:10):
it that it's in a big clump around the galaxy,
mostly concentrated in the middle. We see some clumps out there,
but we don't know the exact structure of dark matter.
But we do sort of know it's density out there, right,
We have some figures for its general density. Yeah, And
it's quite interesting because we think that on average over
the universe, dark matters like eight percent of the matter
of the universe, but in our neighborhood it is actually

(35:33):
a little bit different. The Milky Way, for example, we
think is nine five dark matter. So our whole galaxy
is kind of badly named. It should be called like
the Dark Way or something, the Chocolate Milk Galaxy, the
darklan Milky Way, the Dark Chocolate Way or something. So
we're like nine percent dark matter, which means if we
have like you know, the equivalent of ninety billion times

(35:56):
the mass of the Sun in terms of stars and
gas and all that kind of stuff, that means that
there's like two trillion times the mass of the Sun
in dark matter. It's just so much more. And it's
like twenty times as much dark matter in our galaxy
as normal matter, which is a bigger ratio than the
rest of the universe. Well, that's true for regular matter.

(36:16):
To write like our Milky Way has a higher density
of regular matter than the rest of the universe or
some other parts of the universe. Right, Yeah, that's true.
But on average galaxies have about eighty percent dark matter,
and our galaxy is so there's a big variation. Galaxy
the galaxy and how much dark matter there is. Oh,
I mean our galaxy has more than other galaxies. Absolutely, Yeah,

(36:39):
we are a darker galaxy than most interesting we're more mysterious,
I guess. Yeah. And there's some galaxies out there that
are overwhelmingly dark matter, like nine points something per cent.
And then there are some galaxies that have very little
dark matter. We think that might be evidence of collisions
where dark matter gets separated from the normal matter. All
sorts of crazy stuff. These things tell you the crazy
cosmic history of all of these objects. Interesting. And again

(37:01):
you can tell by when you look at these galaxies
out there. You can tell that they're holding on together
more than they should by the number of stars or
the brightness of them. Right. Yeah, you can tell when
a galaxy is overwhelmingly dark matter because it's stars are
moving super duper fast compared to how bright they are,
And so we can see these faint dwarf galaxies, for example,
just have a handful of stars, but they're whizzing around

(37:23):
in a circle and there's not nearly enough gravity to
hold them in place. Just from the stuff that we
can see, so it's pretty cool, but it's so far away.
How do you know it's not just like filled with
the black holes or something or rocks dark rocks, because
we can see light passing through it, right, we can
see through it to something else behind it. If there
was a black hole there, it would absorb the light.
If it was just like a huge death star or

(37:45):
something cloaked, then it would absorb that light. So we
see it as invisible, not as dark all right, So then, um,
it's sort of danser in our galaxy. What about in
in our more immediate neighborhood or like a star solar
system also extra dark mattery, it's and in our neighborhood.
Remember that while the Milky Way is dark matter that
is spread out throughout the stars, we think, so normal

(38:08):
matter clumps up a lot more than dark matter, which
means that there isn't that much dark matter in any
like cubic light year of space. So in a cubic
light year of space, there's less than a one quarter
of one one thousands of the mass of the Sun.
In a cubic light year of space, that's like a
quarter of the mass of Jupiter in a cubic light

(38:29):
year of space. That's how much dark matter there is.
M t gives you took Jupiter and spread it over
billions of miles, right, it wouldn't be very much. And
you know, if you zoom in, for example, into like
a cubic meter, that's like ten the minus twenty two
grams of dark matter in a cubic meter. So you know,
if you look at the space around you in your office,

(38:50):
for example, then there's just like a super tiny amount
of dark matter, almost hard to measure, but some of
it's there are a few particles. And if you zoom
out to like the whole volume of the Earth, there's
less than a kilogram of dark matter in the volume
of the Earth. Again, these are sort of approximations, right,
because he told me earlier that our ability to resolve
or a resolution of dark matter is pretty bad. So

(39:12):
how do we know like that there isn't the sort
of clump of dark matter just around us right now?
We don't know absolutely, We do not know. We are
not sensitive to these things, so it could be a
lot clumpier than we think these numbers are, assuming that
dark matter is mostly smoothly spread out throughout the galaxy
according to the distribution that we've seen from the radius.
But we absolutely cannot tell if there's like a huge

(39:33):
blob of dark matter that we're sitting in, or if
there's almost no dark matter in our neighborhood. And remember
that we have experiments underground looking for dark matter particles.
They're basically trying to measure how the Earth is moving
through this dark matter wind, and they haven't seen anything.
And one of my favorite explanations is like, well, maybe
we're just happy to be sitting in a bubble that
has almost no dark matter in it, which would make

(39:56):
it impossible for us to detect that dark matter wind.
We just don't know it's ghosting us and avoiding is
physically at the same time. But it's kind of interesting
because I think what you talked about earlier, how like
in the volume of the Earth, there's about one squirrels
worth full of dark matter. Like that's not a lot, right,
and the whole Earth is pretty big, but you only
have one squirrel full of dark matter, And that's why

(40:18):
we can't ever detected gravitationally. You know, we're literally looking
for a squirrel that's hiding inside the Earth, and that's
pretty hard to tell the difference. We can't measure the
number of squirrels on Earth. Using grabic, you'd go nuts.
But yeah, so let's talk about then, now, sort of
dark matter between galaxies, because you know, there's a lot
of space between galaxies and we sort of have a

(40:40):
pretty good idea of the structure of the universe, you know,
the galaxy clusters and superclusters. Is does dark matter also
follow these clusters? We think that mostly does, and again
we think it's sort of the opposite that normal matter
follows the path of dark matter. But it's much harder
to see the things between galaxies because there's much less

(41:01):
light there and there's much less visible objects. Like mostly
we have seen where dark matter is within our galaxy
by following the path of stars, their rotation, their wiggles,
their distortions, all that kind of stuff where there are
in stars like tracers to tell us where things are
between galaxies. So it's much trickier. Yeah, I guess you
you can sort of extrapolate, right what we can see

(41:22):
a little bit around this, then you sort of assume
that that's what's happening maybe in the rest of the
universe sort of, But we know that the galaxies are
very different from the rest of the universe. Like, we
know that there's a huge gravitational well that we are sitting,
and that's why there's a galaxy right here, there's a
big blob of dark matter. What does it look like
between our galaxy and Andromeda? You know, are there strands

(41:43):
of dark matter? How quickly does it pete route? Are
there blobs of dark matter out there without any stars
in them at all? And so one way we can
try to figure that out is to look at how
light from distant galaxies is distorted as it passes through
that space. I see, based can do the gravitational lensing
but with galaxies and look for blobs in between galaxy.

(42:04):
But then these blobs we got to be humongous, right,
those blobs would have to be humongous. And in this
case we use a slightly different technique than we do
for looking at like one specific blob before. What we
were doing is called strong lensing, And that's like, I
want to have a blob of dark matter right between
me and another galaxy, so I can see like a
massive distortion. You can see one galaxy how it's distorted,

(42:25):
and you can use that to measure the massive stuff
between you and other galaxies. If instead you think the
dark matter is sort of spread out, so it doesn't
really distort any individual photon that much. You can do
something called weak gravitational lensing where you look at lots
of galaxies and you look for lots of very small
distortions and you sort of add them up statistically to

(42:47):
get a map for where the dark matter might be
and where it might not be. So you see sort
of like fewer generalized distortions over here and more generalized
distortions over there. It can tell you sort of where
the dark matter is dens or and where it's less dense. Interesting,
you started looking for sort of wiggles in the overall picture.
But how would you know that is dark matter? Would

(43:07):
that be changing? Are you assuming that it changes as
our view of the universe changes. Well, we think we
know what galaxies should look like when they're not distorted,
and so we compare how galaxies look too ideas of
how a galaxy should look when it's not distorted, and
how it should look when it's slightly distorted by dark matter.
And so we can use that to estimate like how
much distortion galaxies have. But it's very very weak. You know,

(43:30):
it's hard to tell the difference between a galaxy that's
undistorted and slightly distorted. And that's why we need like
thousands of galaxies to add this up statistically to get
a sense for where the dark matter is interesting. And
this is like an ongoing thing, right, Like there's people
looking for these ripples in our view of the universe. Yeah,
this is recent. Actually there's a program using a huge
telescope with a massive camera five hundred and seventy megapixels.

(43:53):
It's called the Dark Energy Survey, and this camera is
basically build just to do this, just to look at
all the galaxies and build a huge map. And they've
studied a hundred million galaxies out there, like think about
all the crazy stars and planets and everything that's out there,
a hundred million of those. And they have built a
map of where they think the dark matter is between

(44:15):
galaxies using this weak lensing idea. Because I guess we
can tell how old they are the galaxies, right, and
how far away they are from from us, not just
in the night sky, and so we can build the
three D map right exactly. We know where galaxies are
because we can look at like Type one A supernova
within them. We can measure their brightness, and we can
tell how far away they are based on how bright
they appear to be here on Earth. So we have

(44:38):
this incredible three D map of all the galaxies and
then this thing is taking careful pictures of them to
try to estimate how much each one is distorted, and
then it's comparing that to our idea for where we
think the dark matter should be. We have an idea
for where we think dark matter should be based on
where all the galaxies are. Sort of back that up
to the early universe and say where were the dark

(44:59):
matter are have to have been in order to make
these galaxies end up here and that galaxy end up
there to sort of create the large scale structure that
we see, because we think that mostly where galaxies ended
up depends on where dark matter was. So we have
like a simulation for where we think the dark matter
should be based on our idea of how it all works.
And then we go out of measure and build a

(45:21):
real map of where the dark matter is, and then
we can compare the two and that's when the fund starts. WHOA,
So what have we found? Do they match or are
they very different? They mostly match, like it mostly makes sense.
The dark matter is mostly where we expect, but there
are some deviations. It looks like dark matter sort of
more spread out than we expected. Instead of being in
these like thin strands between galaxies, it tends to be

(45:44):
sometimes in places where you don't expect. It's like spread
out and globbed out more than we expected, more than
our simulations predict. It's just a few percent compared to
our predictions, but it's an important deviation. It means that
like maybe something's going on with that dark matter, or
maybe dark matter is made out of something weird we
didn't understand, or maybe we've made a mistake in building
our map of dark matter. But it's sort of like

(46:06):
at the edge of science. These are very very recent
results that dark matter just wants to be alone between
the galaxy or maybe it got timed out. But I
guess what's surprising is that there is dark matter between
galaxies because there's not much matter out there, so why
wouldn't this dark matter also accumulate regular matter. There is
actually a good bit of regular matter out there is

(46:28):
it's just not glowing like a huge amount of the
atoms in the universe are actually between galaxies and this
intergalactic matter, these streams of stuff, and so there's a
good bit of stuff out there. There's just not enough
density to form stars and planets and galaxies and all
kinds of stuff. And so that's why it's not as
visible because it doesn't glow interesting. But then what what

(46:51):
make some dark matter have a lot of bright matter
in it and some nut the denser blobs of dark
batter did form enough stuff to create galaxies and stars.
The kind of strands we're talking about between galaxies is
a smaller fraction of the amount of dark matter. The
density is not there in order to create the gravitational Well,
you need to attract enough hydrogen to get stars to form.

(47:13):
Pretty cool, so the galaxy still show you where like
the densest blobs of dark matter are in the universe,
Like the dark matter also various in density out there. Absolutely, yeah, alright,
well it sort of sounds like do you sort of
know where dark matter is but maybe not too super
high resolution where you can tell if it's super clumpy
or super smooth. And also we have a pretty good

(47:34):
picture of where it is in the whole universe. Yeah,
and scientists are working very hard to build more and
more sensitive tools to try to use these little gravitational
clues to build as accurate and as localized a map
of dark matter as possible, because the better map of
dark matter we get in our galaxy, the more we
can study what it might be and how it came

(47:54):
to be where it is. Yeah. I guess the big
question now is what are you gonna do when you
find it? Daniel retire. Yeah, like the dog that finally
catches up the car and doesn't know what to do
with it. Oh, we'll find some other mystery to focus on.
You'll come up with another cool name darkest. We always
have the seventy of the universe called dark energy that
we haven't even gotten started on. Yeah, that's another huge mystery,

(48:17):
huge hole in our view of the universe and them.
Hopefully you won't lose your mind trying to find them.
I lost eight years ago. All right, Well, another awesome
reminder that there's still a lot of universe out there
to be discoverned. You know, anyone listening to this could
be the person that comes up with the next great
idea or the next grade concept that makes sense of
all this and helps us find these big mysteries in

(48:39):
the universe. If you're in trance by the concept of
building maps and discovering the way the world is and
how everything looks. Remember, the biggest map of the most
important stuff in the universe is still being drawn. You
can really lose yourself in that search for lost things.
Come join me in the asylum or the basement Lost
and Found department. Well, if they for joining us, we

(49:00):
hope you enjoyed that. See you next time. Thanks for listening,
and remember that Daniel and Jorge explained. The Universe is
a production of I Heart Radio. Or more podcast from
my Heart Radio visit the I Heart Radio app, Apple Podcasts,

(49:22):
or wherever you listen to your favorite shows.