The Big Bang is the theory about the birth of the universe that all scientists agree on.

This event supposedly happened about 13.8 billion years ago.

But how do we know that? And what caused it?

In the early 20th-century, most scientists thought the universe was fixed and that it always had been.

And then, astronomers noticed galaxies, vast groups of stars, moving away from each other.

In the 1920s, the American Hubble and the Belgian Lemaître put forward the theory of an expanding universe.