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Dark Matter — The Universe's Missing Mass · Talking Science

0:00
Welcome to the Deep Dive. Today, we're looking
0:03
at this incredible stack of astronomical research
0:06
to try and answer one really mission -critical
0:10
question. Yeah, and it's a huge one. Right. It's
0:12
basically, how can scientists be absolutely certain
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about the existence of something that they have
0:18
literally never seen, touched, or fully understood?
0:22
It really is. I mean, it's one of the most profound
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puzzles in... all of modern science. We're essentially
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exploring the ultimate invisible elephant in
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the room of our universe. Completely. And I actually
0:35
want to start by asking you, the listener, to
0:36
do something kind of simple. Just take a look
0:38
at your hand for a second. Really look at it.
0:40
Notice the skin, the fingernails, those intricate
0:42
little veins. And now I want you to think about
0:45
the chair you're sitting in. The walls of your
0:47
room, too. Exactly. The walls, the earth outside,
0:50
the sun, even like every single star you have
0:54
ever seen in the night sky. Right. Now, what
0:56
if I told you that every single bit of that,
0:58
like everything you've ever interacted with,
0:59
touched or detected in your entire life is essentially
1:02
just the garnish? It's wild to think about. It
1:05
is because it makes up only about, what, one
1:08
sixth of the actual matter in the universe. The
1:11
other five, six is completely invisible. Yeah.
1:13
And it sounds impossible when you phrase it like
1:16
that. I mean, it completely defies our everyday
1:18
experience. It really does. Because we are so
1:20
wired to believe that if something is real, we
1:22
can see it or or at least, you know, bump into
1:25
it in the dark. OK, let's unpack this because
1:28
we have this fascinating source material in front
1:30
of us and it keeps returning to this term. dark
1:34
matter. Right, dark matter. But dark in a scientific
1:37
context doesn't mean like a dark room, does it?
1:40
I mean, it's not like it's casting a shadow.
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No, far from it, actually. Dark is, well, it's
1:45
a rather poor historical label that just kind
1:47
of stuck. Oh, really? Yeah, it would be much
1:50
more accurate if we called it invisible matter
1:52
or even, you know, transparent matter. Transparent
1:55
matter. Because it gives off absolutely zero
1:58
light. It absorbs zero light and it reflects
2:02
zero light. If you were holding like a baseball
2:05
sized chunk of it in your palm right now, you
2:07
wouldn't see it and your fingers would pass right
2:09
through it. That is just, that's crazy. It is
2:12
utterly transparent and yet the scientific community
2:15
is overwhelmingly confident that it is there.
2:19
heavily threading through every single galaxy,
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including our own Milky Way. Which is just a
2:24
wild concept to accept. I mean, how do you prove
2:26
something is there when you can't see it, touch
2:28
it, or even like bounce a radar beam off of it?
2:32
Well, by looking at what it does to the things
2:34
we actually can see. The story of this discovery
2:38
is really the ultimate master class in how science
2:41
detects the invisible. We find it by mapping
2:44
its effects. Basically, it leaves gravitational
2:46
footprints. Gravitational footprints. I like
2:48
that. Which brings us to the first massive footprint.
2:52
Because to understand how we know this invisible
2:54
stuff is there, we have to look at how things
2:57
move. Exactly. And I want to start with our own
2:59
solar system just as a baseline for how gravity
3:02
is supposed to work. That is the perfect place
3:04
to start. Because you know to understand the
3:07
invisible we have to establish the rules for
3:10
the visible right so In our solar system, we
3:13
have the sun sitting right in the center, and
3:15
it holds all the planets in their orbits with
3:18
its gravity. Because almost all the mass of the
3:20
solar system is concentrated right there in the
3:22
middle. Yes. And because of that, the gravitational
3:25
pull gets weaker and weaker the further out you
3:28
travel. So think of it like standing next to
3:30
a massive bonfire. Oh, yeah. That's a good analogy.
3:33
When you're right up close, the heat is intense.
3:37
But as you step back, the heat dissipates. Exactly.
3:41
And gravity works the exact same way. It follows
3:44
an inverse square law. fading rapidly over distance.
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That is a wonderful way to visualize it. And
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because gravity is the tether that keeps the
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planets whipping around in their orbits, the
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speed of those planetary orbits tells us exactly
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how strong the gravity is at that specific distance.
4:00
Oh, okay. That makes sense. Right. So Mercury
4:01
is practically skimming the sun's surface, comparatively
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speaking. Yeah, it's super close. So it has to
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whip around incredibly fast to avoid falling
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in. It completes an orbit in just 88 days. But
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if you travel way... out to the icy edges of
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the solar system, out to Neptune, the gravitational
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tether from the sun is incredibly weak out there.
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So Neptune is basically taking a leisurely stroll.
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A very leisurely one. I mean, it takes Neptune
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165 years to make a single trip around the sun.
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That's a long year. Right. But that falling off
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of speed with distance is the classic unbreakable
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signature of mass being concentrated in the center.
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Okay, so if we take that exact same logic, and
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we apply it to a whole galaxy, the setup looks
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pretty similar, doesn't it? You would think so,
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yes. Because when you look at a photograph of
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a classic spiral galaxy, it's this blindingly
4:55
bright cluster of stars and glowing gas bunched
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up right in the middle. Exactly. And then it
5:00
just kind of fades and thins out as you reach
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the edges. So naturally, you'd assume the outer
5:05
stars at the edge of the galaxy should be moving
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slowly, just like Neptune. Right, that was the
5:09
unanimous assumption. It made perfect mathematical
5:12
and intuitive sense to everyone. But science
5:15
requires observation. So in the 1970s, an astronomer
5:19
named Vera Rubin decided to actually measure
5:22
those orbital speeds. Wait, how do you even measure
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that? You can't exactly point a police radar
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gun at a star 50 million light years away and
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clock its speed. Oh, no, you can't. But you use
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something very similar in principle, actually.
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It's called the Doppler effect, but with light
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instead of sound. Oh, like when an ambulance
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goes by and the pitch changes? Exactly like that,
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but visual. So as the galaxy spins, the stars
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on one side are moving. toward us and stars on
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the other side are moving away. Yeah. When a
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star moves toward us, its light waves get compressed
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slightly, shifting toward the blue end of the
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spectrum. When it moves away. The light stretches
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out, shifting toward the red end. So by measuring
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that tiny shift in color, Rubin could calculate
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exactly how fast the stars were moving. That's
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brilliant. It is. And what her measurements revealed
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completely broke the accepted rules of physics.
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Let me guess. The outer stars weren't slowing
6:15
down. They weren't slowing down at all. The stars
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way out on the absolute fringes, you know, where
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the visible glow of the galaxy basically fades
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to nothing. Yeah. They were orbiting just as
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fast as the stars much closer to the dense center.
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That is, I mean, that is like finding out Neptune
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is racing around the sun at Mercury speed. Exactly.
6:37
That shouldn't even be physically possible. I
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mean, the gravity from the glowing center of
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the galaxy shouldn't be anywhere near strong
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enough to hold on to those outer stars. if they're
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moving that fast. Right, they should literally
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be flung out into deep space, like a child spinning
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off a merry -go -round that's going too fast.
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So why aren't they flying off? Well, if we connect
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this to the bigger picture, the sheer physics
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of it leaves only one logical conclusion. Okay.
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If those outer stars are moving that wildly fast
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and they aren't flying off into the void, something
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invisible must be holding them in. So there's
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extra gravity coming from somewhere. Yes. There
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must be an absolutely massive amount of unseen
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matter forming a colossal halo around the galaxy,
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extending far, far beyond the glowing stars we
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can capture with telescopes. And its gravitational
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grip is what's whipping those outer stars around.
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Exactly. The galaxies were behaving as if they
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weighed far more than the sum of their visible
7:33
parts. Because there is an unseen anchor. Okay,
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that makes sense. Spinning galaxies are great,
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but... If I'm a skeptic, I'm not rewriting the
7:42
fundamental makeup of the universe based on just
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one spinning anomaly. Sure. Is there anything
7:47
else out there proving this massive halo exists?
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Like, if there is this massive, heavy, invisible
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stuff everywhere, it has to affect something
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other than spinning stars, right? Absolutely.
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You would want an independent line of evidence.
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And to find it, we have to look at how mass affects
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light itself. Okay, now we're getting into the
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really mind -bending stuff. Yeah, this requires
8:06
bringing in Albert Einstein and his theory of
8:09
general relativity. Einstein taught us that mass
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doesn't just act as an invisible tether. It actually
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bends the physical fabric of space. Like placing
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a heavy bowling ball in the middle of a trampoline.
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The fabric just sags around it. Perfect analogy.
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And light travels along that fabric. So if a
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beam of light from a distant object passes by
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something incredibly heavy, like a massive cluster
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of galaxies, it doesn't travel in a straight
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line. Because the line isn't straight anymore.
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Right. It follows the dip in the fabric. The
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heavy object bends the path of the light, acting
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just like a giant cosmic magnifying glass. Or
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kind of like looking through the bottom of a
8:48
thick wine glass. You know, if you slide the
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base of a wine glass over a piece of text, the
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letters warp and stretch and distort. That is
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exactly what astronomers see when they point
8:57
their telescopes at massive clusters of galaxies.
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And we're talking hundreds of galaxies bound
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together by gravity here. So a huge amount of
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mass. Yes. They see the light from even more
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distant background galaxies getting stretched,
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sneered, and twisted into bizarre arcs and rings
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around the cluster. Oh, wow. And this phenomenon
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is called gravitational lensing. Gravitational
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lensing. Okay, so what does this all mean? How
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does a visual smear of light in a telescope translate
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into a measurement of something's weight? It
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basically comes down to the geometry of that
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bend. By measuring the exact curvature of those
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scratched light smears, physicists can calculate
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precisely how strong the lens is. So they can
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kind of reverse engineer it? Exactly. They mathematically
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reverse engineer the distortion to figure out
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exactly how much mass must be sitting there in
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the middle to bend the light by that specific
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amount. And when they crunch those numbers? Every
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single time, the cluster weighs five, sometimes
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six times more than all the visible stars and
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glowing gas can account for. That is wild. The
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light is being heavily warped by something that
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simply isn't shining. The lensing literally weighs
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the invisible. Okay, I have to jump in here and
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play devil's advocate for a second. Because I
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know what you, the listener, are probably thinking
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as you listen to this. Go for it. We are throwing
10:13
around concepts like invisible matter and exotic
10:16
transparent halos. But why does it have to be
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exotic? What do you mean? Well, why couldn't
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this... missing mass just be regular stuff sitting
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in the dark, like, I don't know, dead stars that
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burned out long ago, rogue planets floating without
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a sun, black holes. Or just massive, dense clouds
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of cold gas. Exactly. Stuff that's simply too
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dark for our telescopes to pick up. It is a completely
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sensible, grounded objection. And honestly, it
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is the very first thing the scientific community
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assumed, too. Really? Oh, yeah. They even gave
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it an acronym, MACHOS. which stands for Massive
10:52
Compact Halo Objects. Machos, I love that. It's
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a beautifully reasonable idea, and scientists
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rigorously tested it for years. But the evidence
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ultimately came back with a resounding no. Why,
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though? If this matter is dark, how do we confidently
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know it's not just made of dark atoms? What's
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fascinating here is that the answer doesn't come
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from looking at galaxies today, but from looking
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at the very beginning of everything. The Big
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Bang. Okay, you've got my attention. We actually
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understand the physics of the early universe
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incredibly well. The early universe was a cosmic
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furnace of unimaginable heat and pressure. And
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we can calculate the exact conditions of that
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furnace and figure out exactly how much ordinary
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matter, protons, neutrons, you know, the building
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blocks that make up atoms, was forged in those
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first few minutes before things cooled down.
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Hold on, hold on. How do we have a receipt for
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a cosmic furnace that burned out 14 billion years
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ago? How can we possibly verify that math? By
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looking at the oldest light in the universe.
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It's called the cosmic microwave background.
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Okay. It is literally the afterglow of the Big
11:55
Bang, still echoing through space right now.
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By studying the tiny temperature variations in
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that afterglow and by measuring the exact ratio
12:03
of the simplest elements in the universe, like
12:06
how much hydrogen exists compared to how much
12:08
helium, we can determine the exact density of
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the universe during those first few minutes.
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Oh, wow. So it leaves a permanent signature.
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It does. And that density tells us the absolute
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maximum amount of normal atomic matter the Big
12:21
Bang was physically capable of producing. So
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we literally have a cosmic inventory. And what
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happens when we compare that inventory to the
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total weight of the dark matter we've been measuring
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with those spinning galaxies and bending light?
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The books simply do not balance. Not enough atoms.
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Not even close. If all the dark matter in the
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universe was just dead stars, cold gas, and rogue
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planets, regular atomic matter, we would need
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far more atoms than the Big Bang was physically
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capable of manufacturing. Wow. So it's not just
12:51
a matter of our telescopes being too weak to
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spot it. The math of the universe's birth proves
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it fundamentally cannot be made of the same atomic
12:59
stuff as us. Exactly. It is something entirely
13:02
different, a totally new type of matter. It has
13:06
mass, and therefore it has gravity, but it doesn't
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interact with electromagnetism at all. Which
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means no light. Right. It doesn't absorb or emit
13:12
light, and it barely interacts with normal matter.
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I really want you to pause and visualize this
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right now. Because this isn't just some abstract
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math problem floating out in deep space. No,
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it's very local. Right. Because dark matter is
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threaded through our entire galaxy to hold it
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together. And because it doesn't interact with
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regular atomic matter, it means this invisible
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substance is passing right through you in this
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exact moment. It is. It is passing through the
13:37
room you're sitting in, slipping right through
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the spaces between the atoms in your body, and
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traveling through the entire planet Earth, and
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you don't feel a thing. It is very much like
13:47
a ghost, albeit a very heavy one. A heavy ghost,
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I like that. But because we can't touch it or...
13:52
you know bottle it in a lab there is one final
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incredibly clever counter argument that physicists
13:59
had to wrestle with oh what's that well some
14:03
scientists asked a really provocative question
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what if the invisible matter isn't there at all
14:08
what if our understanding of the universe is
14:11
just flawed oh That is an interesting pivot.
14:13
Like maybe gravity doesn't fade away following
14:16
that inverse square law over massive galactic
14:18
distances. Exactly. They hypothesized that maybe
14:21
the fundamental law of gravity behaves differently
14:23
on a sprawling galactic scale. So gravity itself
14:25
changes. That was the idea. Perhaps that tweaked
14:29
gravity is what's holding onto the outer stars
14:31
and bending the light, basically faking the effect
14:34
of extra mass. It is a very serious idea in physics.
14:38
But how do you test a tweak in the fundamental
14:41
laws of gravity? I mean, you can't put a whole
14:43
galaxy in a laboratory and run experiments on
14:46
it. You can't. But you can look for a natural
14:48
cosmic laboratory. And the universe provided
14:51
us with a spectacular one, an event known as
14:53
the Bullet Cluster. Here's where it gets really
14:56
interesting. Because the Bullet Cluster isn't
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just a group of galaxies, is it? It is a snapshot
15:01
of an incredibly violent collision. It is two
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massive clusters of galaxies crashing straight
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into each other at millions of miles per hour.
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So think of it like this. Imagine two massive
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crowds of people running full speed at each other.
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Okay. Most of the ordinary matter in these galaxy
15:17
clusters isn't actually the stars. It's huge,
15:21
diffuse clouds of hot gas filling the space between
15:24
the stars. Right. The gas is where most of the
15:26
normal mass is. So when these clusters collide,
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those gas clouds slam together. It's like the
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running crowds are all wearing heavy, bulky winter
15:36
coats and carrying huge duffel bags. That's a
15:38
great visual. Thanks. When they hit in the middle,
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they experience immense friction. They tangle
15:44
up, drag, slow down, and pile into a massive
15:48
traffic jam right at the center of the collision.
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But the dark matter. Right. What if half the
15:53
people in those crowds were invisible ghosts?
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They don't have heavy coats. They don't experience
15:57
friction. So what happens to them? The ghosts
16:00
would just walk right through the massive pileup
16:02
unimpeded and keep on running. And that is precisely
16:05
what the observations of the Bullet Cluster revealed.
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Really? Yes. The ordinary matter, the hot glowing
16:11
gas, which we can see using X -ray telescopes,
16:14
slammed together, created immense friction, and
16:17
got stuck in the middle. Just like the crowds
16:19
in winter coats. Exactly. But when astronomers
16:22
use gravitational lensing, That light bending
16:25
trick we discussed earlier, to map out where
16:27
the actual mass was? Mass had moved on. The vast
16:30
majority of the mass had sailed straight through
16:32
the collision without slowing down at all. The
16:34
invisible mass and the visible gas had physically
16:37
separated from one another. The ghosts walked
16:40
right out the other side. They did. This is why
16:43
it's the ultimate smoking gun. Because if there
16:45
is no dark matter... and it's just the visible
16:47
gas exerting a weird, tweaked form of gravity,
16:50
then the center of the gravity should have stayed
16:53
anchored perfectly to the visible gas piled up
16:56
in the middle. Exactly. It forces the conclusion.
16:58
You cannot tweak the math of gravity to explain
17:01
why the gravitational pull is physically separated
17:04
from the stuff that's supposed to be generating
17:06
it. That makes total sense. It proves the mass
17:08
is a physical independent entity that is separate
17:11
from the glowing gas. The evidence is staggering
17:14
when you lay it all out like that. I mean, we
17:16
have the spinning outer stars of galaxies behaving
17:19
terribly. We have massive clusters warping the
17:22
fabric of space and bending light. We have the
17:24
cosmic receipt from the Big Bang. proving it
17:26
isn't made of normal atoms. Right, and we have
17:29
the bullet cluster, showing the mass physically
17:31
separating from normal matter in a collision.
17:34
These are completely independent methods, measuring
17:37
entirely different phenomena across the universe,
17:39
and they all point to the exact same truth. The
17:42
foundation of the universe is built on a substance
17:44
we cannot see. And yet, with all of that overwhelming,
17:49
multi -layered proof... Well, we still don't
17:52
actually know what it is. No, we don't. That's
17:54
the part that just humbles you. We are on this
17:57
subterranean stakeout for a ghost. And we still
18:00
don't have a mug shot. It truly is the great
18:02
frontier of modern physics right now. We know
18:04
how much of it there is. We know where it congregates.
18:07
And we know exactly how it shapes the cosmos
18:09
on a macro scale. But on a micro scale. If you
18:12
ask a physicist what a single dark matter particle
18:14
looks like, they have to shrug. The leading theory
18:16
is that it's a brand new kind of fundamental
18:19
particle, something completely outside our current
18:22
standard model. To clarify for the listener,
18:25
the standard model is basically the physicist's
18:27
periodic table of particles, right? It catalogs
18:30
every tiny building block we know of. Exactly.
18:33
Quarks, electrons, neutrinos. Dark matter isn't
18:36
on that table. It's an entirely undiscovered
18:38
species of particle. So how on earth do you catch
18:42
a particle that is currently passing straight
18:44
through the earth without hitting anything? Very,
18:46
very patiently. Physicists have built these exquisitely
18:49
sensitive detectors. They take vats of ultra
18:53
-pure liquid, like xenon, and they bury them
18:56
deep, deep underground, often in abandoned gold
18:59
mines or beneath mountains. Why do they have
19:01
to go a mile underground? Why not just do it
19:03
in a lab on the surface? To use the Earth itself
19:06
as a shield, the surface of our planet is constantly
19:09
being bombarded by cosmic rays and background
19:11
radiation. Oh, so it's too noisy. Way too noisy.
19:13
If you put a sensitive detector on the surface,
19:16
it would be blinded by the noise of normal particles.
19:18
But down there in the dark, beneath millions
19:20
of tons of rock, these detectors sit in absolute
19:24
perfect silence. Just waiting. They are waiting
19:26
for a lottery ticket event. They are waiting
19:28
for the incredibly rare, statistically minuscule
19:31
chance that a dark matter particle, instead of
19:34
slipping cleanly through the vast empty space
19:36
inside an atom, actually scores a direct hit.
19:38
And bumps into an atomic nucleus. Yes, causing
19:41
a tiny, tiny flash of light. And we have been
19:43
waiting for decades, decades of brilliant scientists
19:47
sitting in these subterranean layers, watching
19:49
the monitors for a single definitive bump in
19:52
the night, and so far, silence. It's true. Which
19:55
has to be intensely frustrating. But, I mean,
19:58
there is also a profound romance to it, isn't
20:00
there? This quiet, generations -long stakeout
20:03
for the invisible foundation of the universe.
20:05
It is exactly how frontier science operates.
20:07
The final answer isn't in yet. But I find this
20:10
to be one of the most wonderful situations in...
20:13
all of human inquiry. How so? Because we aren't
20:15
just making wild philosophical guesses. We have
20:18
rock -solid verifiable data telling us a massive
20:22
truth about reality. It's not a fudge factor.
20:25
It is an honest, precisely stated mystery just
20:28
waiting for the right experiment to crack it
20:30
wide open. That's a great way to put it. Somewhere
20:32
out there, everywhere out there, is the dominant
20:35
form of matter in our cosmos, and we've only
20:37
ever felt its gravitational pull. It is incredible.
20:41
to wrap your mind around, just to summarize where
20:43
we've been today. You look up at the stars, you
20:46
look at your own hands, and you are seeing just
20:48
a tiny, tiny fraction of reality. A very small
20:51
slice. We know dark matters out there because
20:54
galaxies spin far too fast for their visible
20:56
weight because heavy clusters warp light too
20:58
sharply, and because galaxy collisions leave
21:01
the invisible mass running right out the other
21:04
side. The universe is predominantly constructed
21:06
of something we have barely met. Which leaves
21:08
me with one final... Kind of wild thought to
21:11
end on. I want you to mull this over. Oh, this
21:14
should be good. If dark matter is five times
21:17
more abundant than the ordinary matter that makes
21:19
up you, me, the Earth, and the stars, and it
21:23
is passing all around us right now, could there
21:26
be complex structures to it? That is the million
21:28
-dollar question. Right. We know it barely interacts
21:31
with us. But over billions of years, interacting
21:34
with its own gravity, could there be dark matter
21:36
clumps? Could the universe be filled with entirely
21:39
invisible dark matter planets? Or even massive
21:43
shadow galaxies intertwined right here with our
21:45
own, completely oblivious to our existence, just
21:48
as we are to theirs? The math and the sheer depth
21:50
of the mystery leave the door open to so many
21:53
incredible, humbling possibilities. We have really
21:56
only scratched the surface. Something to think
21:58
about the next time you look up at the seemingly
22:00
empty spaces in the night sky. The dark isn't
22:03
empty at all. Thanks for joining us on this deep
22:05
dive. We'll catch you next time.