Reionization Early Galaxies

According to the theory of the Big Bang, the cosmic microwave background was formed when then the temperature cooled so that that electrons and protons could combine to form atoms of neutral hydrogen. Currently however, the inter galactic medium consists mainly of ionized hydrogen gas.  When and how this gas became reionized is one of the biggest puzzles in current cosmology.    See Early star-forming galaxies and the reionization of the Universe in Nature.