Moon formation. Artwork of the formation of the Moon according to the giant impact theory. This is the most widely accepted theory of the Moon's formation. It says that an object about the size of Mars collided with the Earth around 4.6 billion years ago. Material from the outer layers of both bodies would have been thrown into orbit around the Earth, forming a disc. This then coalesced to form the Moon. This theory explains many features of the Moon, such as the lack of volatile substances in its rocks (due to the intense heat generated by the impact) and its lack of an iron core (since it formed from rocky surface material).