A fireball of radiation at extremely high temperature and density, but occupying a tiny volume, is believed to have formed around 13.7 billion years ago. 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
- From a massive explosion, the big bang, the universe started and has been expanding ever since.
- Gamma Ray Bursts are the most violent explosions the universe has seen since the big bang, astronomers say.
- Some of the matter ejected by the big bang forms galaxies, like our very own milky way.
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