The gas cloud, which is visible only with an X-ray telescope, contains more than a hundred galaxies and enough material to make a thousand more. The galaxies in most cases do not contain enough hot gas to be observed with an X-ray telescope, but they do show up in optical photographs. Because the galaxies were discovered before the hot gas cloud, these objects are called galaxy clusters, or clusters of galaxies. Galaxy clusters are formed through the merger of smaller groups and clusters over billions of years. The gas in clusters is thought to have been heated to fifty million degrees or more when the cloud collapsed about five to ten billion years ago.
Galaxy clusters are the most massive gravitationally bound objects in the universe.
In recent years X-ray observations have shown that galaxy clusters must contain large amounts of dark matter to hold the hot gas cloud together. The total mass of the dark matter needed is typically about five times as great as the mass of the gas and the galaxies. Chandra observations may provide insight into what constitutes dark matter and how it is distributed. The gas in the central regions of the 3C295 cluster shows signs that it is cooling and settling onto the central galaxy. Astronomers think that the material falling onto 3C295 over the eons has caused it to grow into its present size and shape.