Galaxy Clusters, Superclusters, and Voids

Most galaxies have been pulled together by gravity into groups. The Milky Way and Andromeda, together with about 20 smaller galaxies, make up a collection of galaxies called the Local Group. Although most galaxies are found in small groups, about 10 percent of the galaxies in the universe are part of larger systems called clusters. Galaxy clusters may contain anywhere from hundreds of galaxies to thousands of galaxies. Astronomers have identified about 10,000 clusters. The Milky Way is located on the outskirts of a cluster called the Virgo Cluster, which consists of about 1,000 galaxies.

Clusters, in turn, are organized into even larger collections called superclusters. Both the Local Group and the Virgo Cluster are members of the Virgo Supercluster.

There are also vast regions of space, known as voids, that are nearly empty of galaxies. Astronomers' understanding of these areas is somewhat fuzzy. Voids could simply be regions of space that were left empty when gravity drew galaxies together to form clusters and superclusters. Or they could be expanses of space that were swept clean of matter by shock waves from supernovae or that never contained much matter to begin with. It is even possible that voids are not really empty. They may be teeming with galaxies that are just too faint to be seen.