The Collaborative International Dictionary of English v.0.48:

big bang theory \big bang theory\ n. (Astronomy, Cosmology)
   The theory that the known universe originated in an explosive
   event (the big bang) in which all of the matter and energy
   of the universe was contained in a single point and began to
   rapidly expand and evolve, starting as high-energy particles
   and radiation, and, as it cooled over time, evolving into
   ordinary subatomic particles, atoms, and then stars and
   galaxies. According to this theory, the four-dimensional
   space-time continuum which we perceive as our universe
   continues to expand to the present time, but it is unknown
   whether the expansion will continue indefinitely or
   eventually stop or even reverse, possibly leading to a
   contraction to a single point sometimes referred to as the
   "big crunch". The competing "Steady-state Theory" gradually
   lost favor in the 1980's and 1990's. See also big bang.
   [PJC]

WordNet (r) 3.0 (2006):

big bang theory
    n 1: (cosmology) the theory that the universe originated
         sometime between 10 billion and 20 billion years ago from
         the cataclysmic explosion of a small volume of matter at
         extremely high density and temperature [syn: big-bang
         theory, big bang theory]