The question Why is the sky dark at night? In 1826, Heinrich W. M. Olbers 1758-1840 drew attention to the fact that, in a large universe of infinite age, filled more or less uniformly with stars, the sky should be a continuous blaze of light, because every line of sight from an observer would ultimately encounter a star. The paradox is that, despite this, the sky is dark at night. The resolution of the paradox lies in identifying an underlying assumption that is wrong. Since the paradox makes assumptions about the properties of the universe, it is clear that the darkness of the night sky is of cosmological importance. The paradox is resolved in the Big Bang cosmology: the finite age of the galaxies means that there has been insufficient time to fill the universe with light. Furthermore, the expansion of the universe leads to yet more dilution of the observed sky brightness from remote objects.