Well, there is. It's called Dark Matter.

Dark matter is spread throughout space, engulfing the Milky Way and all other galaxies.

It's four times more common in the universe than visible matter, yet we've never seen or directly observed it!

So how do we know it's there? Because of gravity.

A galaxy is like a giant merry-go-round that all the stars are riding.

In order to stay on, each star has to hold on tight — that's where gravity comes in — otherwise it'll fly off into space.

The problem is, the force of gravity from all the stars we can see isn't strong enough to hold galaxies together.

All the stars should be flying off the edge of the merry-go-round. But they're not.

It's as if some invisible rope is helping them hold on.

Physicists believe that rope is the gravitational force from stuff we can't see, hence, Dark Matter.

And how do we know the speeds of stars in distant galaxies?

The doppler effect, which is why a police siren shifts to a lower pitch as the car passes, also works for light.

That's how radar catches you speeding, and how astronomers measure the rotation of galaxies.

So, we use what we can see to tell us about what we can't.