There are many wacky theories about how the moon was formed, including one that suggests it's actually a huge, dormant spacecraft. Of the several, slightly more scientific hypotheses, the leading one argues the moon was spun from the debris a collision between ancient Earth and a protoplanet scientists have named Theia. The "giant impact" theory, as it's known, has suffered from a lack of direct evidence, but now a group from a German university claim to have found some. Previous analyses of moon rock found it to be identical to rock on Earth, which disagrees with the theory as we should see chemical traces of the alien planet Theia. Armed with a more sophisticated mass spectrometer, a German team have re-examined moon rock and discovered a measurable difference in the ratio of Oxygen isotopes. In simpler terms, the samples have a slightly higher concentration of Oxygen 17 than Earth rock does, which the researchers claim is evidence that Theia played a part in the formation of the moon.
As the BBC reports, some scientists are a little more skeptical, arguing you'd expect a more radical difference in chemical compositions between Earth and the theoretical Theia. The German team also had only a small sample set to work with, but the lead researcher is pretty convinced: "This confirms the giant impact hypothesis." You can't really prove it either way, seeming as we're thought to be about 4.5 billion years too late to know for sure, but we've shot a query off to our man on the moon base in the hope he might have something to add.