The fact that the night sky is dark even though in an infinite universe with stars that live forever, the night sky would be as bright as the daytime sky. The paradox disappears when it is realized that stars do not live forever. In the modern big bang model, expansion of the universe also plays a role in making the sky at night dark by redshifting the cosmic background radiation to a band well below the visible.
Ascribed to German Astronomer Heinrich Olbers, although the paradox had been previously discussed by many philosophers and astronomers, including Kepler, Halley, and Jean-Phillippe de Cheseaux.