.... is in fact possible.
Hello everyone, first post, I hope I don't make too many mistakes!
Back to creating matter. It happens all the time. Pairs of particle/anti-particle pairs spring into existence from nothing, and no-one bats an eyelid. A simple consequence of Quantum Theory. Of course, they tend to annihiliate each other pretty fast, so as not to violate the uncertainty principle, but in certain circumstances, such as close to the event horizon of a black hole, one of the pair of virtual particles can cross the horizon whilst the other wanders off, becoming "real". Bingo, something from nothing. (Check out any books/websites about "Hawking Radiation" for more on this.)
Well, sort of, anyway. Of course, I have simplified things, but my point is that your comment about a random mass appearing by itself being ludicrous is inherently false.