The extra-galactic background light: A modern version of Olbers' paradox

A non-technical discussion is given of the energy density E of the extra-galactic background light. The fact that E is small means that the space between galaxies is dark, which is a modern version of a classical problem in astronomy known as Olbers' paradox. It is seen that the order of magnitude of E is fixed by the order of magnitude of the lifetime of the galaxies, as pointed out by Harrison; but that the expansion of the Universe can affect E by a smaller factor, typically about 2. These comments should help to end persistent confusion about the effects of the lifetime of the galaxies and the expansion of the Universe on the darkness of the night sky. It is hoped that Olbers' so-called paradox can now rest in peace.