Soon after the Big Bang, the Universe evolved from a hot primordial plasma, which we now observe as the cosmic microwave background (CMB), to a space filled with dark matter, radiation, and neutral gas. During recombination, the Universe entered a starless Dark Age. Over the next several hundred million years, gravity produced a web of cores and interconnecting filamentary structure. As the cores grew, portions collapsed to form the first stars and clusters of stars. Eventually, groups of star clusters surrounded by gas became the first galaxies.