The Universe would be a dull and dark place if the gaseous baryons did not cool, collapse, and form stars and galaxies. However, if this gas is allowed to cool unimpeded at the rate predicted from known atomic physics, in the context of a well-established cosmological model, the gaseous matter would form stars and galaxies with a high e?ciency, so that far more than the observed fraction of about 10 15% of the baryonic matter would be found in luminous stellar systems. Therefore, cooling must be damped or regulatedby heating processes, and observations show that this feedback is a widespread...