Cooling In hierarchical models, structure grows by gravitational collapse of succesively larger structures. The emergence of galaxies as separate entities within this hierarchy depends on the ability of baryons within a given overdense region to cool and form stars before being subsumed into the next higher level of the hierarchy. Cooling of gas in protogalaxies is thought to be a runaway process. Unless this cooling is suppressed, most of the baryonic material in the universe would collapse into dwarf-galaxy size objects. As dwarf galaxies do not appear to be so overwhelming abundant, some mechanism to counteract cooling is needed.
Plausible mechanisms involve both internal and external agents.