The Big Bang was the moment 13.8 billion years ago when the universe began as a tiny, dense, fireball that exploded. Most astronomers use the Big Bang theory to explain how the universe began. But what caused this explosion in the first place is still a mystery.
So, what has happened since the Big Bang?
Since the Big Bang, the universe has been expanding. In the early years, everything was made of gas. This gas, mostly hydrogen and helium, expanded and cooled. Over billions of years, gravity caused gas and dust to form galaxies, stars , planets, and more.
The matter that spread out from the Big Bang developed into everything in the universe, including you. You are made of star stuff!
