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Do you ever think about what would happen

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if the world were a little bit different?

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How your life would be different

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if you were born 5,000 years from now

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instead of today?

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How history would be different

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if the continents
were at different latitudes

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or how life in the Solar system
would have developed

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if the Sun were 10 percent larger.

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Well, playing with these
kinds of possibilities

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is what I get to do for a living

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but with the entire universe.

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I make model universes in a computer.

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Digital universes that have
different starting points

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and are made of different amounts
of different kinds of material.

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And then I compare
these universes to our own

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to see what it is made of
and how it evolved.

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This process of testing models
with measurements of the sky

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has taught us a huge amount
about our universe so far.

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One of the strangest
things we have learned

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is that most of the material
in the universe

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is made of something
entirely different than you and me.

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But without it,

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the universe as we know it wouldn't exist.

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Everything we can see with telescopes

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makes up just about 15 percent
of the total mass in the universe.

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Everything else, 85 percent of it,

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doesn't emit or absorb light.

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We can't see it with our eyes,

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we can't detect it with radio waves

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or microwaves or any other kind of light.

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But we know it is there

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because of its influence
on what we can see.

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It's a little bit like,

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if you wanted to map
the surface of our planet

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and everything on it

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using this picture of the Earth
from space at night.

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You get some clues
from where the light is,

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but there's a lot that you can't see,

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everything from people
to mountain ranges.

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And you have to infer what is there
from these limited clues.

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We call this unseen stuff "dark matter."

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Now, a lot of people
have heard of dark matter,

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but even if you have heard of it,

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it probably seems abstract,

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far away, probably even irrelevant.

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Well, the interesting thing is,

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dark matter is all around us

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and probably right here.

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In fact, dark matter particles

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are probably going through
your body right now

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as you sit in this room.

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Because we are on Earth

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and Earth is spinning around the Sun,

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and the Sun is hurtling through our galaxy

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at about half a million miles per hour.

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But dark matter doesn't bump into us,

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it just goes right through us.

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So how do we figure out more about this?

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What is it,

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and what does it have to do
with our existence?

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Well, in order to figure out
how we came to be,

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we first need to understand
how our galaxy came to be.

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This is a picture of our galaxy,
the Milky Way, today.

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What did it look like
10 billion years in the past

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or what would it look like
10 billion years in the future?

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What about the stories

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of the hundreds of millions
of other galaxies

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that we've already mapped out
with large surveys of the sky?

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How would their histories be different

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if the universe was made of something else

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or if there was more or less matter in it?

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So the interesting thing
about these model universes

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is that they allow us
to test these possibilities.

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Let's go back to the first
moment of the universe --

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just a fraction of a second
after the big bang.

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In this first moment,

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there was no matter at all.

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The universe was expanding very fast.

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And quantum mechanics tells us

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that matter is being created and destroyed

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all the time, in every moment.

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At this time, the universe
was expanding so fast

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that the matter that got created
couldn't get destroyed.

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And thus we think that all of the matter
was created during this time.

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Both the dark matter

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and the regular matter
that makes up you and me.

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Now, let's go a little bit further

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to a time after the matter was created,

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after protons and neutrons formed,

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after hydrogen formed,

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about 400,000 years after the big bang.

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The universe was hot and dense
and really smooth

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but not perfectly smooth.

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This image, taken with a space telescope
called the Planck satellite,

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shows us the temperature of the universe

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in all directions.

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And what we see

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is that there were places
that were a little bit hotter

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and denser than others.

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The spots in this image

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represent places where there was
more or less mass in the early universe.

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Those spots got big because of gravity.

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The universe was expanding
and getting less dense overall

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over the last 13.8 billion years.

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But gravity worked hard in those spots

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where there was a little bit more mass

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and pulled more and more mass
into those regions.

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Now, all of this
is a little hard to imagine,

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so let me just show you
what I am talking about.

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Those computer models I mentioned
allow us to test these ideas,

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so let's take a look at one of them.

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This movie, made by my research group,

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shows us what happened to the universe
after its earliest moments.

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You see the universe
started out pretty smooth,

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but there were some regions

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where there was
a little bit more material.

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Gravity turned on
and brought more and more mass

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into those spots that started out
with a little bit extra.

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Over time,

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you get enough stuff in one place

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that the hydrogen gas,

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which was initially well mixed
with the dark matter,

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starts to separate from it,

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cool down, form stars,

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and you get a small galaxy.

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Over time, over billions
and billions of years,

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those small galaxies crash into each other

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and merge and grow
to become larger galaxies,

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like our own galaxy, the Milky Way.

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Now, what happens
if you don't have dark matter?

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If you don't have dark matter,

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those spots never get clumpy enough.

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It turns out, you need at least
a million times the mass of the Sun

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in one dense region,

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before you can start forming stars.

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And without dark matter,

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you never get enough stuff in one place.

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So here, we're looking
at two universes, side by side.

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In one of them you can see

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that things get clumpy quickly.

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In that universe,

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it's really easy to form galaxies.

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In the other universe,

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the things that start out
like small clumps,

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they just stay really small.

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Not very much happens.

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In that universe,
you wouldn't get our galaxy.

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Or any other galaxy.

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You wouldn't get the Milky Way,

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you wouldn't get the Sun,

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you wouldn't get us.

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We just couldn't exist in that universe.

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OK, so this crazy stuff, dark matter,

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it's most of the mass in the universe,

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it's going through us right now,
we wouldn't be here without it.

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What is it?

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Well, we have no idea.

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(Laughter)

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But we have a lot of educated guesses,

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and a lot of ideas
for how to find out more.

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So, most physicists think
that dark matter is a particle,

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similar in many ways to the subatomic
particles that we know of,

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like protons and neutrons and electrons.

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Whatever it is,

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it behaves very similarly
with respect to gravity.

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But it doesn't emit or absorb light,

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and it goes right through normal matter,

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as if it wasn't even there.

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We'd like to know what particle it is.

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For example, how heavy is it?

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Or, does anything at all happen
if it interacts with normal matter?

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Physicists have lots of great ideas
for what it could be,

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they're very creative.

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But it's really hard,

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because those ideas span a huge range.

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It could be as small
as the smallest subatomic particles,

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or it could be as large
as the mass of 100 Suns.

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So, how do we figure out what it is?

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Well, physicists and astronomers

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have a lot of ways
to look for dark matter.

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One of the things we're doing
is building sensitive detectors

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in deep underground mines,

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waiting for the possibility

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that a dark matter particle,
which goes through us and the Earth,

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would hit a denser material

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and leave behind
some trace of its passage.

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We're looking for dark matter in the sky,

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for the possibility
that dark matter particles

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would crash into each other

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and create high-energy light
that we could see

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with special gamma-ray telescopes.

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We're even trying to make
dark matter here on Earth,

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by smashing particles together
and looking for what happens,

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using the Large Hadron
Collider in Switzerland.

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Now, so far,

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all of these experiments
have taught us a lot

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about what dark matter isn't

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(Laughter)

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but not yet what it is.

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There were really good ideas
that dark matter could have been,

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that these experiments would have seen.

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And they didn't see them yet,

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so we have to keep looking
and thinking harder.

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Now, another way to get a clue
to what dark matter is

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is to study galaxies.

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We already talked about

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how our galaxy and many other galaxies
wouldn't even be here

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without dark matter.

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Those models also make predictions

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for many other things about galaxies:

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How they're distributed in the universe,

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how they move,

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how they evolve over time.

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And we can test those predictions
with observations of the sky.

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So let me just give you two examples

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of these kinds of measurements
we can make with galaxies.

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The first is that we can make
maps of the universe with galaxies.

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I am part of a survey
called the Dark Energy Survey,

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which has made the largest map
of the universe so far.

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We measured the positions and shapes
of 100 million galaxies

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over one-eighth of the sky.

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And this map is showing us all the matter
in this region of the sky,

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which is inferred by the light
distorted from these 100 million galaxies.

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The light distorted from all of the matter

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that was between those galaxies and us.

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The gravity of the matter is strong enough
to bend the path of light.

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And it gives us this image.

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So these kinds of maps

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can tell us about how much
dark matter there is,

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they also tell us where it is

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and how it changes over time.

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So we're trying to learn
about what the universe is made of

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on the very largest scales.

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It turns out that the tiniest
galaxies in the universe

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provide some of the best clues.

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So why is that?

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Here are two example simulated universes

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with two different kinds of dark matter.

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Both of these pictures
are showing you a region

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around a galaxy like the Milky Way.

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And you can see that there's a lot
of other material around it,

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little small clumps.

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Now, in the image on the right,

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dark matter particles are moving slower
than they are in the one on the left.

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If those dark matter particles
are moving really fast,

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then the gravity in small clumps
is not strong enough

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to slow those fast particles down.

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And they keep going.

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They never collapse
into these small clumps.

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So you end up with fewer of them
than in the universe on the right.

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If you don't have those small clumps,

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then you get fewer small galaxies.

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If you look up at the southern sky,

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you can actually see
two of these small galaxies,

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the largest of the small galaxies
that are orbiting our Milky Way,

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the Large Magellanic Cloud
and the Small Magellanic Cloud.

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In the last several years,

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we have detected a whole bunch more
even smaller galaxies.

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This is an example of one of them

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that we detected
with the same dark energy survey

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that we used to make maps of the universe.

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These really small galaxies,

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some of them are extremely small.

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Some of them have as few
as a few hundred stars,

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compared to the few hundred
billion stars in our Milky Way.

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So that makes them really hard to find.

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But in the last decade,

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we've actually found
a whole bunch more of these.

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We now know of 60 of these tiny galaxies

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that are orbiting our own Milky Way.

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And these little guys
are a big clue to dark matter.

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Because just the existence
of these galaxies tells us

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that dark matter
can't be moving very fast,

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and not much can be happening
when it runs into normal matter.

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In the next several years,

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we're going to make much more
precise maps of the sky.

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And those will help refine our movies

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of the whole universe
and the entire galaxy.

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Physicists are also making new,
more sensitive experiments

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to try to catch some sign
of dark matter in their laboratories.

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Dark matter is still a huge mystery.

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But it's a really exciting time
to be working on it.

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We have really clear evidence it exists.

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From the scale of the smallest galaxies

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to the scale of the whole universe.

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Will we actually find it
and figure out what it is?

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I have no idea.

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But it's going to be
a lot of fun to find out.

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We have a lot of possibilities
for discovery,

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and we definitely will learn more
about what it is doing

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and about what it isn't.

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Regardless of whether we find
that particle anytime soon,

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I hope I have convinced you

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that this mystery is actually
really close to home.

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The search for dark matter

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may just be the key to a whole new
understanding of physics

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and our place in the universe.

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Thank you.

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(Applause)