[MUSIC] Joan Cartan-Hansen, Host: Have you ever looked up at the space between the stars and wondered if there was anything there?

Well, that's how scientists think!

Most of what we know about the universe is because scientists have learned it through observation.

And what have they seen?

Light from the stars, dust clouds, and planets, can be observed by using tools that measure light on the electromagnetic spectrum.

And what can't they see?

There are also things like black holes.

They don't emit light, but we know they exist because we can observe how they affect the space around them.

And scientists recorded a lot of matter in the universe that affected the gravity on the things around it, but it wasn't giving off any light, so they decided to call it dark matter.

So like black holes, even though scientists can't see dark matter, they know it exists because of its gravitational influence on things we can see.

Scientists don't really know what dark matter is, but they estimate it makes up about 85% of the universe, so all that space, isn't just space.

For more information about galaxies, check out the Science Trek website!

You'll find it at science trek dot org.