Supercomputer Simulation Sheds Light on Moons Origin || NASA's Ames Research Center

It is widely believed today that the Earth's moon is the result of a giant impact between Earth, then in its early proto-planet stage, and an astronomical body called Theia. The debris from this impact, collected in an orbit around Earth to form the Moon. Analysis of samples brought back from the NASA Apollo missions suggest support for this theory, which is called the Giant-impact theory.
Most moons in our solar system are believed not to have formed this way but from gravitational capture by the planets during the formation of our solar system. Read More at the UK National History Museum.