Galaxy Cluster

The cluster Abell 1689
Credit: NASA Hubble

Gravity is powerful in space. It allows planets to orbit stars, and stars to orbit around galaxies. It also brings galaxies together. There are many groups, and clusters of galaxies in space, which are bound to each other by gravity. 

Galaxy groups have less than 50 galaxies and are 3 to 7 million light-years across. The mass can be 10,000,000,000,000 (10 trillion) times that of the Sun. The Milky Way is part of the Local Group of galaxies, which has around 40 members.

Galaxy clusters are even bigger. They can have between 50 and 1,000 galaxies. Reaching 7 to 35 million light-years across, clusters can contain 10 to 100 times more mass than galaxy groups. 

If you were to look at a cluster for a very long time, you would see that the galaxies in it were all swarming around a central point.