THE Virgo cluster(1-10) is the nearest large concentration of galaxies to us, and therefore plays a critical role in calibrating the cosmological distance scale(11). Unfortunately, the cluster is very irregular, making it difficult to model the distribution of mass within it, and therefore impossible to determine its content of dark matter. X-ray observations offer a way around this problem, by revealing the hot, diffuse gas that is thought to fill the cluster’s gravitational potential well in a way that closely matches the total distribution of mass(12). Here we report X-ray images of Virgo, obtained with the Rosat observatory, which show hot luminous gas extending over most of the optically visible cluster. The data show that a large part of the mass of the cluster is centred on the galaxy M87, with smaller concentrations around M86 and M49. Our results for M86 support the recent suggestion(3) that it is part of a small group of galaxies that is merging with the main cluster.