"Billions of years of expansion mean that the temperature of radiation in space will steadily fall, halving every time expansion doubles the scale of the universe. This expansion continually lowers the density of matter in the universe so that today there is only about one atom in every cubic meter of space on average. The density is so small and the radiation temperature so low after billions of years of expansion that there is too little material in the universe to illuminate the sky around us . . . So the sky is dark at night because of the great age of the universe. The night sky in the universe was once bright, as bright as the surface of the Sun all over the sky, when the universe was about a quarter of a million years old and the expansion was about a thousand times less advanced than it is today. The temperature was too high for any stars, planets, molecules or atoms to exist. No observers could witness that bright night sky."
-The Book of Universes by John D. Barrow. Chapter "Post-Modern Universes".