The X-ray-emitting hot gas that fills clusters of galaxies contains most of the baryonic mass. It cools on timescales shorter than the universe's age, so it must form a "cooling flow" in which gas cools, loses pressure, and flows toward the center where star formation occurs. Long ago, X-ray spectroscopy did not detect the expected cooled gas, and it was postulated that an unknown power source reheated it. The origin of the energy that stopped the flow from cooling remains unknown, as is the eventual fate of this matter.
I will discuss the “Hidden Cooling Flow” scenario, in which the cooling flow occurs, but foreground material blocks our view. This has major implications for mass recycling and star formation in some of the largest structures in the Universe.