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.
While our optical view of our own galaxy, the Milky Way,
is limited by interstellar dust to just a modest fraction of the distance across the Galactic disk, at radio wavelengths emissions from interstellar gas in its various forms can be readily observed throughout the Galaxy.
The Milky Way