An artist's depiction of a collision between two planetary bodies. Such an impact between the Earth and a Mars-sized object likely formed the moon.

The moon is so big that for a long time scientists could not explain its origin; however, recent research has led us to the "Big Splash" theory.

This theory purports that four and a half billion years ago, a planet as large as Mars ran into the young Earth at high speed. The core of the unlucky planet merged with the Earth's own core, while its outer layers were shredded and escaped into Earth's orbit, where they eventually coalesced and formed our moon.