Birth of The Moon

There are basically three famous theories for it. 

The first is the Capture Theory, which says that the moon was a passing body, which the Earth pulled towards itself by gravity. This is how Mars got its moons, Deimos and Phobos, too. The primary problem with this theory is that such moons are irregularly shaped, whereas our moon is somewhat spherical.

The second is the Co-Formation Theory, according to which the moon formed at the same time as the Earth. In the early solar system, gravity pulled floating rocks and other particles together to form both the Earth and the moon. Or two massive bodies collided and the crash resulted in the formation of the earth, with a disk of orbiting objects that then formed the moon.

The third and most widely accepted theory is the Giant Impact Hypothesis. Simply put, it says that a giant Mars-sized body called Theia collided with our planet, which ejected large chunks into the orbit, and gravity pulled them together to form the moon.