text: why is the sky dark at night?   this is a classic question that puzzled people for centuries, known as obler's paradox. if there are so many stars in the sky, why is the night sky not lit up from all the stars?   there are several interesting aspects to this question:   1. first, distant stars are receding from us at increasing speeds (hubble's law). the speed of these distant stars causes their light to be doppler-shifted out of the visible range, so our eyes cannot see the light from these stars any more.  2. the universe is full of dark matter, which does not shine, and probably absorbs light, so a lot of light gets absorbed this way.   3. if a distant star is receding at close to the speed of light, its light might not ever reach us, or might take a very long time to reach us, so the light has not got here yet from some stars.   4. there are black holes in the universe, that are objects so dense that photons are drawn in by the immense gravitational forces, and cannot escape. these also reduce the amount of light that we can receive.   5. there are vast empty areas of space, so there are not an infinite number of stars available to produce light to brighten our nighttime sky.   there are probably some other considerations as well, but this at least gives you a starting point when considering this very interesting question.