The Big Bang is the prevailing cosmological theory of the early development of the universe. Cosmologists use the term Big Bang to refer to the idea that the universe was originally extremely hot and dense at some finite time in the past and has since cooled by expanding to the present diluted state and continues to expand today. The theory is supported by the most comprehensive and accurate explanations from current scientific evidence and observation.[1][2] According to the best available measurements as of 2010, the initial conditions occurred around 13.3 to 13.9 billion years ago.[3][4]