The Big Bang Theory is the leading explanation about how the universe began. The scientific evidence of the theory is the expansion of the universe based on Hubble's law (as indicated by the redshifts of galaxies), and the discovery of the cosmic microwave background.
Steady-state theory is a view that the universe is always expanding but maintaining a constant average density, with the matter being continuously created to form new stars and galaxies at the same rate that old ones become unobservable as a consequence of their increasing distance and velocity of recession. However, it is disproved when quasars are discovered in the 60s.
