The '''Big Bang theory''' is a scientific theory used to account for the acceleration of matter throughout the Universe.
Big bang theories are actually a ''class'' of scientific models that describe the Universe as expanding from a very hot, dense state approximately 13.7 billion years ago<ref>[http://map.gsfc.nasa.gov/m_uni/uni_101age.html WMAP Cosmology 101: Age of the Universe]</ref> (although this number has changed several times throughout recent history). It was first proposed by the Catholic priest [[Georges-Henri Lemaitre]] and evidence for the expansion was observed by [[Edwin Hubble]]<ref>Hubble, E. Proceedings of the National Academy of Sciences of the United States of America, Volume 15, Issue 3, pp. 168-173.</ref>. Later [[George Gamow]] predicted that the Big Bang would leave an observable [[cosmic microwave background|microwave background radiation (or CMBR)]]. This radiation was allegedly discovered by Arno Penzias and Robert Wilson at [[Bell Labs]]; they claimed that it was found to be close to that predicted by Gamow (Gamow predicted a background radiation level equivalent to a roughly 3 K black body object, and the observed level is that of a 2.725 K body).
The term "Big Bang" implies an explosion of matter into pre-existing space, but the theory actually indicates that space is dynamic and more space is constantly created in the interstices between particles as the density of the universe falls. In other words, the Big Bang describes the expansion of space and time. Big Bang theorists state that the Hubble [[redshift]] is a consequence of this stretching of the fabric of space.