Galaxies are divided into two classes – actively, star forming spiral galaxies and red and “dead” elliptical galaxies. All massive galaxies host a supermassive black hole (SMBH) at their centers. In elliptical galaxies, these SMBHs drive energetic outbursts, powered by accreting matter, that govern the formation of new, young stars. While the central supermassive black hole likely plays a major role in controlling star formation, the black hole makes up only a very small fraction of the galaxy’s mass. In a massive elliptical galaxy, most of the mass lies in a dark matter halo, that is filled with hot (ten million degree) X-ray emitting gas. On larger scales, galaxies lie in groups or clusters, where the space between the galaxies is filled with very hot (hundred million degree), X-ray emitting gas. Although the mass of this diffuse hot gas is much larger than the mass of all the galaxies in the cluster, about 85% of the mass in galaxy clusters is dark matter. We discuss how X-ray observations of the hot gas, combined with optical and radio data, allow us to understand the outbursts of SMBHs and to trace the distribution of dark matter.