Theia

http://www.bbc.com/news/science-environment-27688511
http://en.wikipedia.org/wiki/Giant_impact_hypothesis
http://www.designntrend.com/articles/15026/20140605/researchers-find-evidence-of-planet-that-crashed-into-earth-forming-the-moon.htm
http://www.cbc.ca/news/technology/moon-rocks-hint-at-planet-theia-that-once-crashed-into-earth-1.2666306
https://www.youtube.com/watch?v=yukSqUkKj7c
http://www.ibtimes.com/study-links-moons-formation-collision-between-early-earth-theia-planet-sized-body-1595285