Introduction

On the largest scale, the distribution of matter in the Universe is uniform, but on an intermediate level, galaxies are found in gravitationally bound aggregates. These ‘groups’ and ‘clusters’ exist in sizes ranging from a few galaxies to 10 000 galaxies. The gravitational potential which binds galaxies in a cluster also binds a cloud of hot gas which fills the space between and around the galaxies. This gas, the intracluster medium (ICM), has a temperature of tens of millions of degrees. It coexists with the galaxies and, although very diffuse, is a strong source of X-ray emission.

This hot gas was discovered unexpectedly in 1971 through the analysis of X-ray observations. Modern observatories have now measured the X-ray luminosities of hundreds of galaxy clusters, and the morphology of emission from many brighter clusters has been well mapped. The shapes of the gravitational potentials of these clusters have been derived and the mass of X-rayemitting gas determined. (The deeper the gravitational potential well, the faster the motion of the galaxies within the cluster and the greater the concentration of hot gas at the centre.) The mass of hot gas is typically 3–10 times greater than the mass derived from the visible luminosity of the galaxies.

The cluster gravitational potential which fits both X-ray and optical measurements requires the existence of a large hidden mass.