Artist's picture of the giant impact that is thought to have formed the Moon
The giant impact hypothesis is that the Moon was created out of the debris from a collision between the young Earth and a Mars-sized protoplanet. This is the favoured scientific hypothesis for the formation of the Moon.[1]
Evidence for this hypothesis comes from Moon samples which show that:
the surface of the Moon was once molten
the Moon's apparently relatively small iron core and a lower density than the Earth, and
evidence of similar collisions in other star systems (which result in 'debris disks')
The colliding body is sometimes called Theia for the mythical GreekTitan who was the mother of Selene, the goddess of the moon.[2][3]
There are several unanswered issues with this hypothesis. Lunar oxygen isotopic ratios are essentially identical to Earth, with no evidence of a contribution from another solar body.[4] Also, lunar samples do not have expected ratios of volatile elements, iron oxide, or siderophilic elements (chemical elements which bond with iron), and there is no evidence that the Earth ever had the magma ocean implied by the hypothesis.
↑Halliday, Alex N. 2000. Terrestrial accretion rates and the origin of the Moon. Earth and Planetary Science Letters176 (1): 17–30. doi:10.1016/S0012-821X(99)00317-9.