  There is compelling evidence that black holes (BHs) in cluster centers
vigorously interact with their surroundings, indicating that any realistic
model of cluster formation needs to account for these processes. Here we use
high-resolution cosmological simulations of a large cluster and group sample to
study how BHs affect their host systems. We focus on two specific properties,
the halo gas fraction and the X-ray luminosity-temperature scaling relation,
both of which are notoriously difficult to reproduce in self-consistent
hydrodynamical simulations. We show that BH feedback can solve both of these
issues, bringing them in excellent agreement with observations, without
alluding to the `cooling only' solution that produces unphysically bright
central galaxies.
