The current explanation for the origin of the universe, in which it expands and evolves from an initial very high-temperature condition about 13.7 billion years ago. The expansion time is given from the reciprocal of the Hubble constant (the rate at which galaxies are receding). All all-pervasive background radiation of 3 K is considered to be residual from the big bang and is the strongest supporting evidence for the theory.

http://www.big-bang-theory.com/ An overview of the big bang theory.