The Big Bang is a cosmological model of the universe that has become well supported by several independent observations. After Edwin Hubble discovered that galactic distances were generally proportional to their redshifts in 1929, this was taken to indicate that the universe is expanding. If the universe is seen to be expanding, then it must have been smaller, denser, and hotter in the past. This idea has been considered in detail all the way back to extreme densities and temperatures, and the resulting conclusions have been found to conform very closely to what is observed. The term 'Big Bang' was first coined by Fred Hoyle, ironically, in a derisory statement seeking to belittle the credibility of the theory which he did not believe to be true. However, the discovery of the cosmic microwave background in 1964 was taken as almost undeniable support for the Big Bang.

Analysis of the spectrum of light from galaxies reveals a shift towards longer wavelengths proportional to each galaxy's distance in a relationship described by Hubble's law, which is taken to indicate that space-time is undergoing a continuous expansion. Furthermore, the accidental discovery of cosmic microwave background radiation in 1964 suggests that the universe has cooled from an initial hot, dense state as a result of the expansion of space-time. The discovery of the cosmic microwave background led to general acceptance among physicists, astronomers, and astrophysicists that the Big Bang describes the evolution of the universe quite well, at least in general terms.

Further evidence supporting the Big Bang model comes from the relative proportion of light elements in the universe. The observed abundances of hydrogen and helium throughout the cosmos closely match the calculated predictions for the formation of these elements in the first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.

However, there are mysteries of the universe that are not explained by the Big Bang model alone, such as the similarity of regions of the universe which, within the scope of the model, have never been causally connected. Alan Guth's 1981 theory of Cosmic Inflation, a short, sudden burst of extreme exponential expansion in the very early universe, provided an explanation for several of the features unaccounted for by the original Big Bang model. The successor to Guth's original theory has found some circumstantial support, but it is not yet nearly as well supported as the Big Bang model.