A model for the history of the universe, according to which it began in an infinitely compact state and has been expanding ever since. This apparent beginning occurred between 13 and 20 billion years ago and has come to be known as the Big Bang. The theory is now widely accepted since it explains the two most significant observations in cosmology: the expanding universe and the existence of the cosmic background radiation. The known laws of physics can be used to project backwards and calculate what the universe was like at various stages of its development since 10-43 seconds after the Big Bang. For the first million years, the matter and energy in the universe formed an opaque plasma, sometimes called the primeval fireball. By the end of this period, the expansion of the universe caused the temperature to fall below 3,000 K so that protons and electrons could combine to form hydrogen atoms. At this stage, the universe became transparent to radiation. The density of matter then exceeded that of radiation, where previously the situation was the reverse, and so dictated the rate of expansion of the universe. The microwave background is all that remains of the greatly cooled radiation from the early universe. The first galaxies did not begin to form from the primordial clouds of hydrogen and helium until one or two billion years later. The term big bang may be applied to any model of an expanding universe that had a hot, dense past. See also: steady-state theory.