According to the Big Bang theory, the Universe began very hot and small, and has been rapidly expanding and cooling off ever since. The background radiation from the Big Bang shifted into wavelengths longer than those we can see and the matter (electrons, protons and neutrons) combined into cold, neutral hydrogen and helium atoms, which did not emit visible light. Thus, in the period from about 1 million years to a few 100 million years after the hot Big Bang the Universe was a "sea of darkness". There were no sources of visible light until the first stars and galaxies formed toward the end of that period of darkness.
