When we think about what our universe is made
up of, we automatically think of stars, planets, galaxies! But the reality is that is less than five
percent of the mass of the universe. The other ninety five percent? Well that’s one of the biggest questions
in science today, and what some of the greatest minds in astrophysics are trying to decipher. The hunt for dark matter has spanned decades
and though we can’t see it, smell it, feel it, taste it, or hear it, we can see its gravity
impacting other things. So if we only really know five percent of
the story, discovering this elusive dark matter would unlock an entirely new understanding
of everything and everyone in our known universe. So how close are we to finding dark matter? If you’re not really familiar with dark
matter, let me get you up to speed on what we know so far. What we know is it exists. And that’s about it. No, seriously. But there are a few things we are at least
confident about: Dark matter is this really mysterious, strange
substance. It's amazing in some sense because it's all
around us. Right now, as I'm sitting here, a wind of
dark matter is going through me. It doesn't interact with me, which, at least,
it doesn't interact with me much so my body doesn't realize it's there, the Earth doesn't
realize it's there. We are pretty sure it's a new particle. If it were not a new particle, if there's
something totally crazy, then science would be revolutionized overnight in a way that
it has never been in the history of mankind. And then finally we know a lot about what
dark matter isn't. So there have been lots of different kinds
of experiments that have looked and tried to discover what dark matter is and haven't
found it yet. And so we've excluded a lot of possibilities. But why is this missing piece of our universe
so important to find and understand? It could be that someday if we could manipulate
this stuff, we could actually use it as a source of energy or something. In the far future, there might even be a huge
payoff for us if we could manage to do that. In the late 1900s when JJ Thompson discovered
the electron in cathode ray tubes nobody knew what the electron was good for. He just thought this was an interesting thing
to study. And now when we think about how we live our
lives, we all go around all the time with our heads in our phones, which are packed
full of devices that rely on the quantum mechanical properties of the electron. And so although we haven't found dark matter
yet, there's a huge amount of it out there. And understanding its quantum mechanical properties,
who knows how it's going to change our lives? This is a puzzle that is being put forward
by the universe to us. I cannot think of anything else better to
do with my time than to try to answer that puzzle. Even though we haven’t seen it yet, there’s
a few key pieces of evidence we have found so far in the search for dark matter that
tell us we’re on the right track: We are certain that dark matter exists because
the evidence for additional mass in the galaxy is all over the place in astronomical observations. You can see the gravitational pull on stars
and on galaxies, to even a lot more subtle things like you can see the relativistic effect
of invisible clumps of dark matter causing light rays to bend. Most recently there have been beautiful observations
using optical data and X-ray data looking at gravitational lensing to infer the distribution
of matter in colliding clusters of galaxies. But by far the smoking gun for dark matter
is the Cosmic Microwave Background. This is the earliest thing that we can see
through our telescopes through light. It’s basically a photograph of a moment
in history after the Big Bang, when the universe was only three hundred and eighty thousand
years old. It also shows temperature data, and when we
measure the fluctuations in temperature, the position of the peaks can determine the ingredients
of our universe. It shows that less than five percent of the
total mass of the universe is made up of what we call “normal matter,” like visible
stars, planets, and galaxies. Then twenty six point eight percent of the
mass of the universe is dark matter and the rest is made up of dark energy. If you asked me what dark matter was I'd say
I have no idea. If you ask me what dark energy is, you wouldn't
be able to show that because it would be I have no bleep bleep bleep bleep idea. Which means we should just leave dark energy
for another day. Now, the reason the CMB was so significant
in proving dark matter exists is because when we compare theoretical models with these peaks,
there's an extremely compelling match, practically ruling out a universe without dark matter. So basically putting it all together, dark
matter is the simplest explanation we have that explains all of the data that we have
from different types of observations. To match these discoveries and observations,
scientists came up with a theory for what dark matter could be: WIMPs. WIMPs — which stands for weakly interacting
massive particles, but of course, the name WIMP is so cute that everybody likes to use
it instead — are particles that are heavy, and that's where the “massive” comes from
and “weakly interacting” means that they have an interaction strength that's maybe
around the electroweak force. WIMPs started being discussed sometime in
the 1980s. They've really been dominating the conversation,
I would say, until about the last five or 10 years. WIMPs are beautiful because they solve a lot
of problems kind of for free. You don't add too much, you just get a lot
of explanations for mysteries that we want to know. If we can find WIMPs, it’s possible that
that would then mean we have found dark matter. So scientists began planning and building
a lot of different experiments to look for WIMPs, dispersed all over the world. Experiments look for dark matter in three
ways. You can make it, break it, or shake it. So the experiments that make it try to produce
dark matter particles in ultra high energy collisions of proton beams and accelerators
like in the Large Hadron Collider. And those experiments look for some evidence
that dark matter particles were produced and flew out of the detector. The Large Hadron Collider is pushing particles
together at such high speeds that when they slam into each other, the kinetic energy that
breaks off can be frozen into matter to be studied. It’s possible that these tests could generate
something that matches the properties of dark matter. The second search method is called indirect
detection — the “break it” method. This is when we observe dark matter in space,
and since it is so far away from us, we are only seeing what is produced when dark matter
particles are annihilating each other — which could happen if there’s a high enough density
of them. And finally, the “shake it” method is
actually called direct detection — because scientists theorize that dark matter may set
off extremely sensitive detectors. I work on the DEAP experiment. And we use a detector which has three and
a half tons of argon and is located a mile underground in Sudbury Ontario, Canada. And what we're looking for is some evidence
that a dark matter particle struck an argon atom and then that argon atom deposited the
energy in the detector. And that produces a flash of light. It takes months or even years to get the experiments
going and they often run for month or years just collecting the data that they need in
order to see if the dark matter is there where they think it might be. So with all of these different searches, and
all these different methods, have we found anything close to WIMPs? We've been looking for almost 25 years. And we haven't seen it yet. That means that WIMPS are still allowed to
be the answer. On the other hand, when you don't find something for a few years, then
you start to think, "All right, well, maybe I'd to look in other places too." I think what we're seeing now is a push in
that direction. In fact, the search for dark matter is experiencing
a major, exciting shift right now for the first time in decades. The search for WIMPs will continue, but scientists
are clamoring onto the scene with new ideas for what dark matter could be, bringing the
hunt to new corners of the universe. Once you go beyond the idea of WIMPs and start
thinking about other ideas for what dark matter could be, you find actually there's a lot
of great possibilities. There are very light-mass particles like sterile
neutrinos, which is kind of a cousin of the neutrinos that are part of the standard model,
or axions, this is kind of a very, very, light particle that explains certain mysteries for
the strong nuclear force. One thing I'm actually really
excited about is looking for dark matter through new forces. I think this is an avenue that is a relatively
modest mathematical change to the theory, that opens up a whole new range of different
experimental handles. You could imagine that it's actually something
to do with extra dimensions of space where at every point in space there's a different
direction that we can't see because our eyes don't know how to look in it, but you can
actually send energy into it and you can have particles that live in it. I'm really interested right now in the possibility
that dark matter might actually interact with electrons. And so I'm focusing on looking at searches
using an alternate signal. And I think that that's kind of a trend for
a lot of experimentalists: is we're thinking about how can our experiments do more? How can we test other possibilities? These are just a handful of new theories — there
are so many more, with great names like fuzzy dark matter. Or, what if there was a periodic table of
dark matter just like what we have for elements? Testing these new theories can be folded into
existing experiments or drive completely new ones. Other ideas are actually even more exotic. You could imagine that you're going to use
gravitational waves to discover something about dark matter. Other advances are more in the way that we
actually do analysis. So, there are things like machine learning
that allow us to be very sensitive to tiny and very subtle signals that are buried in
complicated backgrounds. The machines can be trained to discover things
that your eye wouldn't be able to pick out from a set of data. So to recap: We know it exists. We’ve been searching primarily in 3 different
ways for WIMPs, but haven’t found them yet. In doing so, we’ve eliminated a lot of what
dark matter is not — which is progress. And we have a bunch of new ideas to explore. So how close are we to finding dark matter? If the current round of experiments are going
to be able to discover dark matter, and there's a good reason to think they might be, I think
we would know in a couple of years. On the other hand, if these experiments are
not quite what we need, it could take longer than that. I'm an optimist. I'll tell you I could find it tomorrow. I might have already found it. I've got two years of data in the can that
I haven't look at yet. It may be right around the corner. The honest answer is we really have no idea,
but it's exciting because we have a fighting chance of being really close. It is a hugely exciting scientific question
and there are great experiments taking data now. And big leaps forward in sensitivity. So I think the chance that we discover dark
matter in the next 10 years is good. I'm betting on it. When the discovery comes, it's going to be
like a bolt of lightning and it's going to change everything. Thanks for watching and let us know in the comments what topics you’d like us to investigate in future videos. If you want to watch more “How Close Are We?”, be sure to check out our our full playlist and don’t forget to like, share, and subscribe.
