Some scientists are starting to believe that our moon is actually the result of a mid-air space collision of two moons. They say that the two-moon theory could explain why each side of the moon is so different from the other.
It's widely accepted that the Moon was created when a Mars-sized body hit Earth which blasted material that would eventually form the Moon into space. This conveniently explains the differences between the Moon and Earth. However! The differences between the near side of the Moon and the far side (or dark of the Moon) can't be neatly explained in this theory. That's why scientists believe that a smaller, littler Moon had also existed after the Mars-sized body hit the Earth. The little Moon and real Moon co-existed peacefully until the smaller moon was pulled from its orbit and smashed into real Moon, creating our Moon. Moons, moons, moons!
Specifically, the scientists say that the "companion moon, 1/3 the diameter of the Moon, striking at subsonic velocity, does not form a crater. The impact produces an accretionary pile rather than a crater." That means most of the material from little Moon would have stayed on one side, like while the material of the original Moon stayed on the other side. Two Moons, Moon smash, our Moon. [Nature, Space, Ars Technica]