The Moon is a weird satellite as it is way too large by comparison to the moons of other planets we know of. It also sits at a near perfect distance to completely block out the sun whenever it is in alignment with it. And the fact it is ringing like a bell when a heavy object strikes it makes some people believe the moon is hollow, and was created instead of naturally formed.
Who did it? I am not sure we will ever know, but I am open to the possibility that it indeed was placed here to keep the Earth stable. Heck, we might even have done this ourselves, once we evolved enough on the unstabiblized world. Invent time travel and then make it back to the past to make our little blue marble safer for those living on it.