Galaxy clusters are groups of tens to thousands of galaxies that are bound by gravity. These clusters have gigantic sizes of millions of light years. Galaxy clusters contain not only galaxies but also very hot gas (10 million to 100 million degrees!) which emits X-rays. The mass of this gas is generally two to three times larger than the total mass of galaxies in the cluster.
However, astronomers believe that most of the mass of a cluster (about 90%) consists of an invisible matter that acts on its surroundings through gravitation only. This matter, still unknown, is called dark matter.