BIG BANG
Most astronomers believe the Universe began in a Big Bang about 14 billion years ago. At that time, the Universe was inside a very small bubble. It was very hot and dense. Then it exploded. The time, space and matter all began with the Big Bang.
The universe went from very small to larger than a galaxy. And it kept growing. It is still expanding today.
As the Universe expanded and cooled, energy changed into particles of matter and antimatter. These two opposite types of particles destroyed each other.
But some matter survived. More stable particles called protons and neutrons started to form when the Universe was one second old.
The next three minutes, the temperature dropped below 1 billion degrees Celsius.
Now protons and neutrons can join forming hydrogen and helium nuclei
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