The Big Bang theory is a scientific explanation for the origin and evolution of the universe. According to the theory, the universe began as a singularity, an infinitesimally small, infinitely hot, and infinitely dense point. About 13.8 billion years ago, this singularity experienced an immense explosion, which caused it to rapidly expand and cool, forming the universe as we know it today.

As the universe expanded and cooled, matter began to condense and form into galaxies, stars, and planets. The Big Bang theory explains the observed expansion of the universe, the cosmic microwave background radiation, and the abundance of light elements like hydrogen and helium, among other things. It is widely accepted by scientists as the most plausible explanation for the origin and evolution of the universe.