X-ray clusters of galaxies as tracers of structure in the Universe

Clusters of galaxies are visible tracers of the network of matter in the Un iverse, marking the high-density regions where filaments of dark matter joi n together. When observed at X-ray wavelengths these clusters shine like co smic lighthouses, as a consequence of the hot gas trapped within their grav itational potential wells. The X-ray emission is linked directly to the tot al mass of a cluster, and so can be used to investigate the mass distributi on for a sizeable fraction of the Universe. The picture that has emerged fr om recent studies is remarkably consistent with the predictions for a low-d ensity Universe dominated by cold dark matter.