Galaxies and the Expanding Universe
Gravity draws galaxies together into groups known as galactic clusters, which orbit around a common centre. As well as the stars, gas and dust that make up the galaxies, the galactic cluster is embedded in vast clouds of extremely hot gas, filling the space between the galaxies. Scientists, however, have calculated that the galaxies and the gas on their own do not have sufficient mass to allow the cluster to hold together. They deduce, therefore, that dark matter must be present in order to provide sufficient mass to produce the gravitational pull required.
The Milky Way is part of a galactic cluster known as the Local Group. The Milky Way and the Andromeda Nebula are the only two bright members of the Local Group. Other members include the Milky Way’s two small companion galaxies, the Large and Small Magellanic Clouds. Most galaxies are members of clusters containing up to thousands of galaxies.