The moon likely exists due to a head-on collision between a much-younger Earth and a so-called "planetary embryo" known as Theia.

The moon came into being as a result of a head-on collision between a much-younger Earth and a so-called "planetary embryo" known as Theia, according to researchers at UCLA. 
The crash itself, which occurred some 4.5 billion years ago, has already been known for some time. Scientists had believed, however, that the angle of impact was more akin to a sideswipe. But according to findings recently published in the journal Science, a head-on collision is much more likely to have been the cause.
According to a write-up of the findings, "The researchers analyzed seven rocks brough