The moon is thought to have formed around 4.51 billion years ago, shortly after the Earth itself.

Here's the most widely accepted theory:

* The Giant-Impact Hypothesis: A Mars-sized object, called Theia, collided with the young, still-forming Earth. The impact ejected a massive amount of debris into orbit around Earth, which eventually coalesced to form the Moon.

While this is the leading theory, there are other possibilities, and ongoing research continues to refine our understanding of the moon's origins.