Thirteen billion years ago our universe was dark. There were neither stars nor galaxies; there was only hydrogen gas left over after the Big Bang. Eventually hydrogen atoms began to clump together to form stars—the very first ones to exist—initiating a major phase in the evolution of the universe, known as the Epoch of Reionization, or EoR.
“The first light of the universe might imply the existence of an important population of more exotic objects like faint quasars, X-ray binary stars, or perhaps even decaying/annihilating particles.” –Harvard-Smithsonian Center for Astrophysics