A galaxy cluster is a group of galaxies bound together by the force of gravity. Like the planets in our solar system, galaxies in a cluster orbit a common center of mass. However, galaxies execute more complicated orbits than the planets because there is no massive central body in the cluster playing the role of the Sun in our solar system. Galaxy clusters typically contain a few hundred galaxies, and are several megaparsecs (ten million light-years) in size. The orbital velocities of galaxies in clusters provide strong evidence for dark matter.