This visualization shows how ultraviolet light from the first stars and galaxies gradually transformed the universe. Hydrogen atoms, also called neutral hydrogen, readily scatters UV light, preventing it from traveling very far from its sources. Gradually, intense UV light from stars and galaxies split apart the hydrogen atoms, creating expanding bubbles of ionized gas. As these bubbles grew and overlapped, the cosmic fog lifted. Astronomers call this process reionization. Here, regions already ionized are blue and translucent, areas undergoing ionization are red and white, and regions of neutral gas are dark and opaque.
Video Credit: NASA/M. Alvarez, R. Kaehler, and T. Abel (2009)