Quasars were much more common in the early universe than they are today. This discovery by Maarten Schmidt in 1967 was early strong evidence against steady-state cosmology and in favor of the Big Bang cosmology. Quasars show the locations where supermassive black holes are growing rapidly (by accretion). Detailed simulations reported in 2021 showed that galaxy structures, such as spiral arms, use gravitational forces to 'put the brakes on' gas that would otherwise orbit galaxy centers forever; instead the braking mechanism enabled the gas to fall into the supermassive black holes, releasing enormous radiant energies. These black holes co-evolve with the mass of stars in their host galaxy in a way not fully understood at present. One idea is that jets, radiation and winds created by the quasars shut down the formation of new stars in the host galaxy, a process called "feedback". The jets that produce strong radio emission in some quasars at the centers of clusters of galaxies are known to have enough power to prevent the hot gas in those clusters from cooling and falling on to the central galaxy.