In the 1920s in California, astronomer Edwin Hubble observed distant galaxies using an extremely powerful telescope. He made two mind-boggling discoveries.

First, Hubble figured out that the Milky Way isn’t the only galaxy. He realized that faint, cloud-like objects in the night sky are actually other galaxies far, far away. The Milky Way is just one of billions of galaxies.

Second, Hubble discovered that the galaxies are constantly moving away from each other. In other words, the universe is expanding. The biggest thing that we know about is getting bigger all the time.

A few years later, Belgian astronomer Georges Lemaître used Hubble‘s amazing discoveries to suggest an answer to a big astronomy question: “How did the universe begin?”

This breakthrough idea later became known as the Big Bang Theory.