A galaxy cluster or cluster of galaxies is a structure that consists of anywhere from hundreds to thousands of galaxies bound together by gravity. They are the largest known gravitationally bound structures in the universe and were the largest known structures in the universe until the 1980s when superclusters were discovered. One of the key features of clusters is the intracluster medium or ICM. The ICM consists of heated gas between the galaxies and has a temperature on the order of 7-9 keV.