Cosmic Cities

A galaxy cluster is a collection of hundreds to thousands of galaxies bound together by gravity. The Milky Way belongs to the Local Group, a smaller galaxy group of about 80 galaxies spanning 10 million light-years. The nearest large cluster is the Virgo Cluster, containing over 1,300 galaxies about 54 million light-years away.

Galaxy clusters are the largest gravitationally bound structures in the universe. The Coma Cluster contains over 1,000 large galaxies and thousands of smaller ones. Its total mass is about 700 trillion solar masses — but most of that mass is not in galaxies.