About

Galaxies are one of the most important kinds of cosmic "ecosystem:" large systems of stars, dust, and gas, which each appear to sit within a large halo of dark matter (which does not interact with light directly). Cosmic history after the hot Big Bang phase (in which the universe was filled by dense, hot plasma) is largely a history of how dark matter forms large, ellipsoidal halos, in which gas within those halos forms stars, and in which the resulting galaxies merge with each other to form larger galaxies within larger halos. We ourselve live in a spiral galaxy, the Milky Way galaxy, where most stars are concentrated in a flat disk. Other types of galaxy are shaped like giant ellipsoids, or have irregular shapes. Most galaxies have a supermassive black hole in the center. Learning about galaxies is an important piece of the puzzle that is astronomy literacy.