The Epoch of Reionization

During the dark ages, that lasted between 370,000 to 150 million years after the Big Bang, the universe continued to expand and cool. Stars, black holes, and galaxies began to form. We call the space around and in between galaxies the Inter Galactic Medium or IGM for short. The universe was filled with neutral hydrogen and its temperature and density were decreasing. It was assumed to have stayed that way. But the discovery of the first quasar in the early 1960s, changed that view.