Why Is The Night Dark?

This looks quite as a silly question. The sun is down at night, that's why. Yes, but actually on a clear starry night the sky should be luminous like during the day. Why? Because in the universe there are billions of galaxies with trillions of stars. Many of these stars are bigger and shine with brighter light than our own sun. So why is the night sky dark? The paradox can be explained this way: Numerous galaxies and their stars are so old and so far away from us that their light still did not reach us. Another reason for the blackness of the night sky is related to the expansion of the universe. As the galaxies and stars move away from us at extremely high speed, their lights are stretched and transformed into infrared light which transcends the visible spectrum and therefore is imperceptible to the human eye.