The Big Bang theory concerns the creation and early development of the universe. In modern cosmology, it describes a universe that began in an extremely hot, dense state and has been expanding and cooling for roughly 13.8 billion years.
The name is often misunderstood as describing an ordinary explosion in empty space. Instead, the theory says that space itself has expanded, carrying galaxies apart. It does not by itself explain why the universe began, or what—if anything—preceded the earliest state described by current physics.
Belgian physicist and priest Georges Lemaître proposed an expanding-universe model in the 1920s and later described a “primeval atom.” British astronomer Fred Hoyle popularized the phrase “Big Bang” during a 1949 BBC radio discussion, although he favored the rival steady-state theory.
Evidence for the model includes the observed expansion of galaxies, the cosmic microwave background discovered in 1965, and the measured abundance of light elements such as hydrogen and helium. The theory describes cosmic evolution, not a religious account of creation.