The Big Bang Theory is the explosion of dense matter that, according to current cosmological theories, marked the origin of the universe.

In the beginning, a fireball of radiation at extremely high temperature and density, but occupying a tiny volume, is believed to have formed. This expanded and cooled, extremely fast at first, but more slowly as subatomic particles condensed into matter that later accumulated to form galaxies and stars. The galaxies are currently still retreating from one another. What was left of the original radiation continued to cool and has been detected as a uniform background of weak microwave radiation.