Big Bang

The evolution of the Universe.
Credit: NASA

The Big Bang is the explanation for how the Universe started. Astronomers think that everything started in a single point. Then everything expanded and stretched to grow into the huge Universe we see today. 

But why do we think this? 

Well, we know from observing the light from distant galaxies that they have been moving apart over time. If you could wind time backwards, it would mean that the galaxies would have been much closer together in the past. If you go back far enough, all the galaxies, and indeed everything in the Universe would have been in one place.

You can think of the start of the Universe as everything suddenly starting to "fly apart". A bit like an enormous explosion. This is what astronomers call the Big Bang.

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