An Israeli-French team of scientists studying the moon at the University of Bordeaux may have at last uncovered the story behind its formation. Previously, most academic circles supported the “giant impact” hypothesis, which conjectured that the moon formed from one of several collisions between asteroids and Earth roughly 4.5 billion years ago (1). Initially, people widely believed that the composition of the unknown mass that collided with Earth was vastly different from that of our planet. However, according to the model used in this recent study, which re-created the gradual shaping of the solar 
system over time, the composition of the smaller mass was probably very similar to Earth’s (1).
The research offers an alternate explanation for why the Earth and moon share such similar chemical makeups: According to the model, the moon did not break off from the Earth during a collision between the Earth and another planet but rather broke off from the second, unknown mass. Consequently, the scientists believe that the original mass from which the moon broke off contains Earth-like qualities as well, explaining why the Earth and the moon are far more similar than any planet pair in our solar system.
Dr. Hagai Perets, an astrophysicist and one of the researchers involved in the study, said “The most exciting and surprising thing was to find out that we can shed new light on a 30-year-old mystery. Compositionally similar planet-impactor pairs are not rare at all” (2). In all likelihood, the mass that crashed into Earth so many years ago was probably a sister planet, a view that previously was thought to have only a 1% probability (1).
Based on these recent findings, the “giant impact” theory, although dominant for some forty years, may now need revision.
References
1: Another step for mankind: Israeli scientists advance theory on moon’s origin. (n.d.). Retrieved April 13, 2015, from http://www.timesofisrael.com/another-step-for-mankind-israel-scientists-advance-theory-on-moons-origin/
2: Mastrobuono-Battisti, Alessandra, Hagai B. Perets, and Sean N. Raymond. “A Primordial Origin for the Compositional Similarity between the Earth and the Moon.” Nature 520.7546 (2015): 212-15. www.nature.com. 9 Apr. 2015. Web. 13 Apr. 2015.