Big Bang
The leading scientific model for the origin of the universe — an extraordinary expansion of space itself starting ~13.8 billion years ago.
The Big Bang is the cosmological model describing the universe's origin and evolution. Approximately 13.8 billion years ago, the observable universe expanded from an extremely hot, dense state. Crucially, the Big Bang was not an explosion *into* pre-existing space — it was the rapid expansion of space itself.
Four major lines of evidence support the model: the redshift of distant galaxies (Hubble's law), the cosmic microwave background, the observed abundance of light elements, and the large-scale structure of galaxies.
The model does not address what (if anything) preceded the initial state, or what caused the expansion. Active extensions include cosmic inflation and theories about dark energy, which appears to be accelerating the universe's expansion today.