We are used to thinking of the universe as a hot place, full of bright stars, quasars, gamma ray bursts and so on, emanating from a giant explosion - the big bang. However, the universe can also be a surprisingly cool place. It is permeated by a background radiation with a temperature close to that of liquid helium. In this talk, Prof. Roger Blandford will try to explain how this coolness comes about and its implications for what we see around us.