There's a big difference between the side of the Moon we can see, and the side we can't. Although it seems pretty pockmarked to the layperson, "our" side of the Moon is actually the smooth half. On the dark side, there's huge mountain ranges and much bigger craters.
There are lots of theories that seek to explain this disparity. The newest: Earth once had two moons. And the smaller of the pair eventually crashed into its larger sibling on the side that faces away from Earth. From the BBC:
Dr Martin Jutzi from the University of Bern, Switzerland, is one of the authors of the paper. He explained: "When we look at the current theory there is no real reason why there was only one moon.
After spending millions of years "stuck", the smaller moon embarked on a collision course with its big sister, slowly crashing into it at a velocity of less than three kilometres per second - slower than the speed of sound in rocks.
... In a commentary, Dr Maria Zuber from the Massachusetts Institute of Technology (MIT) in Cambridge, US, suggests that while the new study "demonstrates plausibility rather than proof", the authors "raise the legitimate possibility that after the giant impact our Earth perhaps fleetingly possessed more than one moon".
In other words, this isn't so much a proven thing, as the scientific equivalent of a plot bunny. The researchers hope to inspire studies that would either prove them wrong, or lend credence to their ideas. This could end up being the start of something big. Or it could eventually be regarded with about as much respect as the suggestion, "What if Moon were cookie?" We'll have to wait to find out.