It is predicted that all the objects in the Universe that we can see are outnumbered some 30 times by invisible material, or dark matter.
The first evidence for dark matter was discovered by Fritz Zwicky in the 1930s. He observed clusters of galaxies that were moving so fast, that they should break up. But some unseen matter must be pulling back them together.
Another piece of evidence came from Abell 2218, a galaxy cluster that is some 3 billion light-years away. It is causing passing light to bend (known as gravitational lensing). The amount that the light is bending would require the gravity to be 10 times stronger than the visible galaxies can provide. This means that there must be an unseen material that is adding to the total mass.
It is believed that 80% of dark matter consists of subatomic particles such as neutrinos or WIMPs (Weakly interacting massive particles). The other 20% is thought to be massive objects such as planets or brown dwarfs (stars that didn't quite form) that are invisible because they are dark. These are thought to exist in a halo that surrounds galaxies.