Animation of the birth and evolution of the universe. The universe started with the Big Bang, some 13.7 billion years ago. For around 380,000 years it was an opaque fog of ions, which became transparent as electrons and protons combined to form neutral atoms. These atoms formed the first stars and galaxies, around 200 millions years after the Big Bang. Radiation from the first stars re_ionised the gas in the universe, but by then expansion has made it so rarefied that it is not opaque.