The intra-cluster medium (ICM) is a hot (tens of millions of K) and extremely tenuous, X-ray emitting gas that exists between galaxies in clusters of galaxies. In big clusters, the ICM may contain more material than all the galaxies put together (though both are out-weighed 10:1 by dark matter). One of the puzzles in astrophysics has been to explain why this gas hasn't cooled down, because of its X-ray emission, and then condensed to form more galaxies. The solution to this now seems tied to jets emitted from massive black holes that lie at the center of active galaxies at the heart of many galaxy clusters. The black holes swallow up any gas coming close to them and liberate enormous amounts of energy in the process. This energy drives very narrow outflows of gas at velocities close to the speed of light, which reheat the intra-cluster gas. Effectively, the black holes act as thermostats. As hot gas in a cluster cools, it flows to the cluster center and is consumed by the black holes inside active galaxies. Some of the energy from this process drives jets into the cluster gas further out, which heats the remaining gas and drives it away from the cluster center. As the black hole runs out of fuel, it shuts down, ready for the whole cycle to begin again.