How it works

What is in a cluster	How it is seen	How much
Galaxies	Ordinary light	Hundreds to thousands of them, and the smallest share of the mass
Hot gas between the galaxies	X-rays	More mass than all the galaxies together
Dark matter	Only by its gravity	About six times as much as the galaxies and gas together

Why the gas is so hot. Gas falling into a cluster picks up enormous speed, and when clusters and groups run into each other the gas is squeezed and shocked. All that motion ends up as heat. The gas is very thin, far thinner than the best vacuum on the Earth, but a cluster is so large that there is a great deal of it.

Why the galaxies in the middle are yellow ovals. In the crowded centre of a cluster most galaxies are ellipticals, with old stars and little gas left to make new ones. Spirals like ours are commoner further out. See Kinds of galaxy.

The Bullet Cluster. The X-ray picture in this lesson shows two clusters that have run straight through each other, about 3.8 billion light years away. The bright cone on the right is the gas of the smaller one. In the crash the two clouds of gas dragged on each other and slowed down. The galaxies, with wide gaps between them, mostly sailed through. When astronomers mapped where the mass is, using the bending of light, they found it with the galaxies, well ahead of the gas. So most of the mass is neither gas nor galaxies, and it does not drag on anything. That is the plainest evidence there is that dark matter is real stuff, and not a mistake in the law of gravity. See Dark matter and dark energy.

Groups, clusters, superclusters. A group has a few dozen galaxies. A cluster has hundreds or thousands and is held together for good. Superclusters are larger still, but they are loose gatherings that the expansion of the universe is pulling apart. See The cosmic web.