I am at a gold mine a couple hours outside of Melbourne, because, one kilometer underground, they're putting in a detector
to look for dark matter. Let's go. (epic music) It's gonna take 30 minutes to go down a kilometer underground. Dark matter is thought to make up 85% of all the matter in existence. It could form a shadow universe
five times more massive than everything we can see. Over the past several decades, over 50 experiments have tried to make a direct detection of dark matter, but none of them has found anything, except one. Under a mountain in the Italian Alps, there is a dark matter
detector called DAMA/LIBRA. It's been collecting
data for around 20 years and, every year, it sees
the same peculiar results. The rate of detections
increases to a peak in June and then decreases to
a minimum in November. Some scientists think this could be the first direct
evidence of dark matter. But why would dark matter
create a periodic annual signal? Well, this is our galaxy, or at least what it looks
like with visible light. Astronomers suspect it is surrounded and permeated by a huge
sphere of dark matter, invisible particles
that are zipping around all in random directions. According to most theories, dark matter doesn't interact
with anything including itself, except through gravity. We think there should be five
times as much dark matter as there is ordinary matter. Now, our solar system is
moving around the galaxy at 220 kilometers per second. That means we're also
moving through dark matter at this rate, except earth orbits the sun
at 30 kilometers a second. So, for half the year, we're moving with the sun, going faster through dark matter. And, the other half the year, we're moving in the opposite direction, so going slower through dark matter. And, the idea is we
encounter more dark matter when we're moving through it fastest, which happens to be in June, and less of it when we're moving slowest, which happens in November. The actual geometry is a
little more complicated. The solar system is tilted at 60 degrees relative to the plane of the galaxy, but the idea still works. So, the signal observed at DAMA/LIBRA may be due to this motion
through dark matter or it might not be due
to dark matter at all. It could just be something
mundane, like the temperature, humidity, moisture in the
soil, the snow on the mountain, or the number of tourists in Italy. All of these things fluctuate
with a period of one year. And, that is why they're gonna build an almost identical experiment
in the Southern hemisphere, down the bottom of this gold
mine outside of Melbourne, because, there, the seasons are reversed, but our motion through dark
matter is still the same. So, if we see the same signal, it's pretty strong evidence for the existence of dark matter. One of the big problems
that DAMA/LIBRA has is that there are other
very similar experiments that don't see anything. And, this has led to a lot of uncertainty about is the DAMA/LIBRA
signal really dark matter? So, yeah, we don't know, right? The favorite thing in science... But, why do we think dark matter exists in the first place? In 1933, Swiss astronomer, Fritz Zwicky, was studying the coma cluster, a collection of more
than a thousand galaxies. These galaxies are
gravitationally bound together. So, they all orbit around their
collective center of mass. Zwicky measured the orbital
speeds of these galaxies and found that somewhere moving
way faster than he expected, it was as if there was a lot more matter in the cluster than he could see, pulling everything inwards. So, he proposed the existence
of invisible matter, which he called dunkle materie, the origin of the term dark matter. No one really took this idea seriously, but 40 years later, dark
matter turned up again. Vera Ruben and Kent Ford
observed the motion of stars in the Andromeda Galaxy. And, they expected that the further out
from the center you go, the slower the stars would be orbiting, but this is not what they found. The rotational velocity
stays almost constant with increasing distance from the center. Without additional mass in the galaxy to pull those stars in, they should be flung off into space. The result was the same in other galaxies. Using radio telescopes, Albert Bosma and others
measured hydrogen gas even further out from a galaxy center, but the rotational velocity
still stayed constant. One way to explain this is to posit the existence
of matter we can't see, dark matter, which holds all these galaxies together. So, let's say you have a star and this represents the mass of everything in the center of the galaxy
that's pulling the star in. The star can maintain a stable orbit if it's centripetal force is equal to the gravitational attraction to all the mass in the rest of the galaxy. And, so you can see that at
about a distance of one meter, this is the speed of the orbit. Okay, but what happens if
we add some dark matter? So, this water bottle represents
the matter we can't see, now there is more mass pulling
this star into the middle, which means, at the same orbit, it can now go much faster, and, in fact, it must go
faster to maintain that orbit. And, this explains the observation. This is what we see. (metal clanking) (Derek laughing) By looking at the
rotation speeds of stars, scientists estimate that about 85% of the mass of a galaxy is dark matter. But, there's another way to
explain these observations without invoking dark matter. And, that is to modify
our theory of gravity. What's the supporting
evidence for thinking that the particle idea
is totally misguided and we should actually be looking at a revised theory of gravity? We can either invoke
something we can't see or you just say, well, the
universe is what we can see, and we need a way to explain
what's going on out there. And, the only way we can do that is by modifying the laws of physics. So, when you look at the
outskirts of galaxies, they've got a lot of
centripetal acceleration. Dark matter says that well,
that centripetal acceleration is due to the gravitational
effect of dark matter. Whereas, the people who
like MOND will say, no, that's centripetal acceleration, that's just the fact that
it's now reached this floor and can't get any lower. So, they, they're saying that
there's not additional force due to dark matter, but there's a limit to how
low the acceleration could go. I think the consensus is hugely in favor of it being a physical substance, in that it just seems reasonable that is that it could be
other particles out there that we haven't seen yet. And, there's more evidence. This is the bullet cluster, a site where two clusters
of galaxies collided. Most of the ordinary
mass of these clusters is in the interstellar gas. And, when the collision occurred, the interstellar gas interacted,
heated up, and slowed down. So, you'd expect that most of
the mass of the bullet cluster would be in the middle
where all of this gas is. But, if you use gravitational lensing, the way that gravity bends light, you can actually measure where most of the mass in this picture is. And, it isn't in the middle. It's actually on either side. So, the best way to explain this is that when the clusters collided, all that gas got stuck in the middle, but the dark matter passed right through, creating the most gravitational lensing where we can see the
least ordinary matter. Even more evidence for dark matter comes from the oldest
light in the universe. 380,000 years after the big bang, light could finally travel
through the universe unimpeded. And, this is what we see as the cosmic microwave background or CMB. The red spots show
where the early universe was a little hotter and the blue spots show
where it was a little cooler, but these temperature
differences were tiny. Just 0.01%, but they are there. And, you can turn this
picture into a graph by counting up how many blobs
there are of different sizes. So, there's the most common size blob, which results in this peak, but there are also
other common size blobs. And, so you get these other
peaks of decreasing size. Now, the height of these peaks depends on how much dark matter there is. In a universe without dark matter, the graph looks like this, but as dark matter increases, the amplitudes of even
numbered peaks decreases. To match the measurements of the CMB, we need about five times
as much dark matter as ordinary matter. This figure also agrees with
the amount of dark matter required to explain the
motion of stars in galaxies and the motion of galaxies in clusters. So, the dark matter hypothesis explains a lot of different observations with a simple theoretical framework, that there's some type
of particle out there that only interacts through gravity. But, what is this particle exactly? Well, since we don't know, scientists have proposed a
whole bunch of different things that it could be. And, now we have to try
to go out and find them. The approach differs depending on what you're trying to find. Dama Libra and the detector
at the bottom of the gold mine are looking for WIMPs, weakly interacting massive particles. These particles are expected to weigh about as much as a proton, but interact with ordinary
matter extremely weakly. At the heart of the detector, our seven seven kilogram
crystals of pure sodium iodide. So, that's actually
sodium iodide in there. Yeah. I didn't expect it to be so clear. The idea is that very, very
rarely a dark matter particle may hit a nucleus in the crystal and transfer its energy. This creates a flash of
light called a scintillation, which is detected by
photomultiplier tubes, very sensitive light detectors, which are positioned above
and below each crystal. But, there's a problem, even the purest sodium iodide crystal contains radioactive potassium. And, when a potassium atom decays, it emits an electron and a gamma ray. Now, the electron can cause a
scintillation in the crystal just like the hypothesized
dark matter particle. So, to eliminate these events, the sodium iodide crystals are submerged in a tank full of 12 tons
of linear alkylbenzene. This is a liquid scintillator that emits light when
exposed to a gamma ray and that light can then be detected by photo multiplier tubes in the tank. So, if there's a simultaneous
detection in the crystal and in the tank, it was most likely a potassium decay, not a dark matter event, but there's another problem, cosmic rays. Energetic particles from the sun and other galaxies hit the
top of Earth's atmosphere creating muons, essentially heavy electrons, which stream toward the earth at close to the speed of light. Muons can also create flashes
of light in the crystal. This is Muon detector, and it's got these three
paddles of plastic here separated by some pieces of steel. If we see a flash of light in all three, basically the same time, then we know that Muon
has passed through them. So, if I hit reset, we can see, counting up the muons being seen. So, it's at least a few a second. This is why all sensitive
particle detectors are located deep underground Here, we have the muon detector, now one kilometer underground, and it's been running for
something like 15 minutes, and there have been no muon counts. Yeah, we have to leave this
running for a long time, I think, even if we wanted
to get a single hit. We expect the number of muons down here to be about a million less. And, we didn't see a million at the top, so we're probably not
gonna see any down here. And, this is the whole point of putting a dark matter
detector underground. You wanna get rid of all the background that would create noise in the detector. But, even this shielding is not enough. We all have muon detectors
immediately above the tank. So, if a flash is seen in a crystal at the same time a muon is detected, it can be ruled out. Being underground brings
its own challenges. The walls of the mine
contain trace amounts of radioactive elements
like uranium and thorium, which decay into radon gas. The requirements here are fairly serious for dark matter experiments. We have to completely
control the environment, in particular, the radon level. To counteract this, the walls
of the cavern are coated with special paint to contain
radioactive particles. The crystals are immersed in a continuous stream
of pure nitrogen gas. And, the entire detector is shielded by 120 tons of steel and plastic. Wow, look at the size of that cavern. There is a lot riding on this experiment. It will validate or disprove one of the most contentious
results in physics. So, if we see nothing, well, this is the death of DAMA/LIBRA but if we see something,
well, we are all happy. I actually like the idea that because 80% of the mass of
the universe is dark matter or dark stuff, maybe there's more than just one particle that dark matter is made of. It could be an entire dark
standard model if you like. A dark version of
everything that we can see or maybe something more complex, 'cause there's so much more of it. Like I really hope it's that. - Do you think that dark matter interacts with ordinary matter? - If we want to find
out what this stuff is, we better hope there's
some level of interaction that we can at least probe when it comes to doing experiments. If God gave me the great book of physics and there were two sections,
section A and section B, one for the luminous matter and one for dark matter, and they didn't talk to each other, I would say that was a
very peculiar universe. But, in science, we have to
live with the possibility that, at some level, we
may never find the answer. It may elude us, but at least we tried. The sponsor of this video, Brilliant, is the opposite of dark matter. You see it everywhere and
it's highly interactive. Brilliant is an innovative
stem learning platform that guides you through engaging, hands on courses in math,
science, and computer science. They have great courses on so
many topics from pre-algebra to probability, to computer
science fundamentals. It's the ideal compliment to watching educational YouTube videos. After learning about a topic, you can dive deeper and
get your hands dirty, so you really understand it. For example, if you wanna learn more about dark matter, gravitational lensing, and the cosmic microwave background, I highly recommend their
astrophysics course. A lot of care and attention
goes into all their courses. Every lesson and interactive simulation builds on what you've learned previously. And, every step of the way,
your knowledge is tested through quizzes, which
check your understanding and reinforce what you've learned. If you get stuck, there's
always a helpful hint. So, I encourage you to check
out the courses on offer over at brilliant.org/veritasium. And, I bet you'll find something there that you wanna learn more about. Plus, if you click through right now, Brilliant are offering 20% off an annual premium subscription to the first 200 people to sign up. So, I want to thank Brilliant
for supporting Veritasium and I want to thank you for watching.
