For more than 30 years, the prevailing view of the formation of our moon has been the "giant impact hypothesis".
According to the giant impact hypothesis, our moon formed as the result of the last of a series of "giant impact" mergers between planetary embryos that eventually formed the Earth.
In this last collision, one embryo was nearly Earth-sized and the other approximately Mars-sized.
The precursors to the current four rock planets – Mercury, Venus, Earth and Mars – appear to have been dozens of smaller bodies known as "planetary embryos".