The leading theory says the Moon formed from debris after the young Earth collided with another planet-sized body, Theia, about 4.5 billion years ago. Computer models predicted the Moon and Earth should differ in their isotopes, yet earlier measurements found no difference.
Scientists at the Universities of Göttingen, Cologne and Münster analysed lunar basalts from Apollo 11, 12 and 16 and found a subtle difference in the ratio of oxygen-17 to oxygen-16, about 12 parts per million. The result, published in Science in 2014, supports the giant impact hypothesis. The difference is smaller than predicted, possibly because Earth and Theia formed in the same region of the solar system.
via Moon formed after collision of planets | University of Göttingen