The universe is simple.

In their minds,

all you needed to make it were four elements:

earth,

air,

fire,

and water.

As theories go, it's a beautiful one.

It has simplicity and elegance.

It says that by combining

the four basic elements in different ways,

you could produce all the wonderful diversity of the universe.

Earth and fire, for example,

give you things that are dry.

Air and water, things that are wet.

But as theories go, it had a problem.

It didn't predict anything that could be measured,

and measurement is the basis of experimental science.

Worse still, the theory was wrong.

But the Greeks were great scientists of the mind

and in the 5th century B.C.,

Leucippus of Miletus came up

with one of the most enduring scientific ideas ever.

Everything we see is made up

of tiny, indivisible bits of stuff called atoms.

This theory is simple and elegant,

and it has the advantage

over the earth, air, fire, and water theory

of being right.

Centuries of scientific thought and experimentation

have established that the real elements,

things like hydrogen,

carbon,

and iron,

can be broken down into atoms.

In Leucippus's theory, the atom is the smallest,

indivisible bit of stuff that's still recognizable

as hydrogen,

carbon,

or iron.

The only thing wrong with Leucippus's idea

is that atoms are, in fact, divisible.

Furthermore, his atoms idea turns out

to explain just a small part

of what the universe is made of.

What appears to be the ordinary stuff of the universe

is, in fact, quite rare.

Leucippus's atoms, and the things they're made of,

actually make up only about 5%

of what we know to be there.

Physicists know the rest of the universe,

95% of it,

as the dark universe,

made of dark matter and dark energy.

How do we know this?

Well, we know because we look at things

and we see them.

That might seem rather simplistic,

but it's actually quite profound.

All the stuff that's made of atoms is visible.

Light bounces off it, and we can see it.

When we look out into space,

we see stars and galaxies.

Some of them, like the one we live in,

are beautiful, spiral shapes, spinning gracefully through space.

When scientists first measured the motion

of groups of galaxies in the 1930's

and weighed the amount of matter they contained,

they were in for a surprise.

They found that there's not enough visible stuff

in those groups to hold them together.

Later measurements of individual galaxies

confirmed this puzzling result.

There's simply not enough visible stuff in galaxies

to provide enough gravity to hold them together.

From what we can see,

they ought to fly apart, but they don't.

So there must be stuff there

that we can't see.

We call that stuff dark matter.

The best evidence for dark matter today

comes from measurements of something

called the cosmic microwave background,

the afterglow of the Big Bang,

but that's another story.

All of the evidence we have

says that dark matter is there

and it accounts for much of the stuff

in those beautiful spiral galaxies

that fill the heavens.

So where does that leave us?

We've long known that the heavens

do not revolve around us

and that we're residents of a fairly ordinary planet,

orbiting a fairly ordinary star,

in the spiral arm of a fairly ordinary galaxy.

The discovery of dark matter took us

one step further away from the center of things.

It told us that the stuff we're made of

is only a small fraction of what makes up the universe.

But there was more to come.

Early this century,

scientists studying the outer reaches of the universe

confirmed that not only is everything moving apart

from everything else,

as you would expect in a universe

that began in hot, dense big bang,

but that the universe's expansion

also seems to be accelerating.

What's that about?

Either there is some kind of energy

pushing this acceleration,

just like you provide energy to accelerate a car,

or gravity does not behave exactly as we think.

Most scientists think it's the former,

that there's some kind of energy driving the acceleration,

and they called it dark energy.

Today's best measurements allow us to work out

just how much of the universe is dark.

It looks as if dark energy makes up

about 68% of the universe

and dark matter about 27%,

leaving just 5% for us

and everything else we can actually see.

So what's the dark stuff made of?

We don't know,

but there's one theory, called supersymmetry,

that could explain some of it.

Supersymmetry, or SUSY for short,

predicts a whole range of new particles,

some of which could make up the dark matter.

If we found evidence for SUSY,

we could go from understanding 5% of our universe,

the things we can actually see,

to around a third.

Not bad for a day's work.

Dark energy would probably be harder to understand,

but there are some speculative theories out there

that might point the way.

Among them are theories that go back

to that first great idea of the ancient Greeks,

the idea that we began with several minutes ago,

the idea that the universe must be simple.

These theories predict that there is just a single element

from which all the universe's wonderful diversity stems,

a vibrating string.

The idea is that all the particles we know today

are just different harmonics on the string.

Unfortunately, string theories today

are, as yet, untestable.

But, with so much of the universe waiting to be explored,

the stakes are high.

Does all of this make you feel small?

It shouldn't.

Instead, you should marvel

in the fact that, as far as we know,

you are a member of the only species in the universe

able even to begin to grasp its wonders,

and you're living at the right time

to see our understanding explode.