Big Bang

Big Bang is a term in astronomy for a cosmic event of rapid expansion of matter from a state of extremely high density and temperature that marked the origin of our universe. As per the cosmological theories, a fireball of radiation having extremely high density and temperature was formed sometime around 13.7 billion years ago. It was a tiny fireball, that expanded and cooled, extremely fast at first, but slowed down latter due to subatomic particles started to condense into matter. These formed galaxies and stars. The galaxies are still retreating from one another. The remnant of the original radiation continued to cool to become a uniform background of weak microwave radiation as has been detected.