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What is the Big Bang?

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Quick answer

The Big Bang is the leading explanation for how the universe began — expanding from an extremely hot, dense state about 13. 8 billion years ago and cooling into the galaxies, stars, and planets we see today. It describes the universe's expansion from a tiny, hot, dense beginning.

Key things to understand

  1. It describes the universe's expansion from a tiny, hot, dense beginning.
  2. It wasn't an explosion in space — space itself expanded everywhere at once.
  3. Evidence includes the cosmic microwave background and galaxies moving apart.
  4. The universe is still expanding today.

People also ask

What caused the Big Bang?
Science describes what happened just after the earliest moment; what 'caused' it remains an open question at the frontier of physics.
How do we know the Big Bang happened?
Key evidence includes the faint cosmic microwave background radiation and the observed expansion of the universe.
Was the Big Bang an explosion?
Not in the usual sense — it was a rapid expansion of space itself, not matter flying through pre-existing space.

Part of the guide: Space & Astronomy →