Perhaps you're referring to the theory that the moon is actually and artificial structure so old that it has heen covered by space dust and appears as a planetoid. Ostencibly, we have crashed heavy objects into the moon to create vibrations that we used to measure its density and consistency, and the results were that of a hollow, metal object, as it 'rang like a bell.' As to why 'someone' would put it there, the moon is actually largely responsible for complex life existing on earth. Supposedly the oceans would be unsuitable for anything beyond microbes if it werent for the tidal forces that only exist because of the moon. So this 'artificial moon' theory is most commonly held by pan/tran-spermianists, who believe life on earth is the result of ancient extraterrestrial influence, and it is they who would have put the moon there. This isn't necessarily a theory I subscribe to, just one I think is interesting and perhaps plausible.
Interesting theory. No such thing as a coincidence! I was particularly thinking about that dark side of the moon. We really don't know what goes on there...
