Dark Matter

The force of Gravity depends upon how much mass there is in the system you are looking at. Therefore, even if an object is not making any light, we can still know that it is there if we can see the effect that its gravity has on the things around it.

When astronomers look at the whole Universe they find that most of the gravity is caused by stuff that cannot be seen.

In fact, we know that it cannot be "normal" matter at all (made of atoms like you and me) but must be something completely different!

This is called Dark Matter and scientists are working very hard to try to detect some. This is difficult because, unlike normal matter, it cannot be detected by most experimental apparatus, but simply goes straight through it!