Groups & Clusters of Galaxies
Galaxy clusters are the largest gravitationally bound objects in the universe. They have three major components: (i) hundreds of galaxies containing stars, gas and dust; (ii) vast clouds of hot (30 - 100 million degrees Celsius) gas that is invisible to optical telescopes; and (iii) dark matter, a mysterious form of matter that has so far escaped direct detection with any type of telescope, but makes its presence felt through its gravitational pull on the galaxies and hot gas.
The hot gas envelops the galaxies and fills the space between galaxies. It contains more mass than all the galaxies in the cluster. Although the galaxies and hot gas clouds are very massive, scientists have determined that about 10 times more mass is needed to hold the cluster together. Something, namely dark matter must exist to provide the additional gravity.
Astronomers think that galaxy clusters form as clumps of dark matter and their associated galaxies are pulled together by gravity to form groups of dozens of galaxies, which in turn merge to form clusters of hundreds, and even thousands of galaxies. The gas in galaxy clusters is heated as the cluster is formed. This heating can be a violent process as gas clouds enveloping groups of galaxies collide and merge to become a cluster over billions of years.
Chandra X-Ray Observatory Center