About a billion years after the Big Bang, when the Universe was still very young, stars began to group together. These groups are made of billions of stars and are called Galaxies.
Galaxies travel through space together to form clusters.
We don't know exactly how many galaxies there are in all, but there are probably billions and billions. They have a large centre with two long swirling arms. Looks a little like the letter 'S'. It is thought that they contain much older stars. The Milky Way contains about 250 billion stars, any of which may have orbitting planets like our solar system.