According to Inflation for Beginners,
... quantum physics allows the entire Universe to appear, in this supercompact form, out of nothing at all, as a cosmic free lunch. The idea that the Universe may have appeared out of nothing at all, and contains zero energy overall, was developed by Edward Tryon, of the City University in New York, who suggested in the 1970s, that it might have appeared out of nothing as a so-called vacuum fluctuation, allowed by quantum theory.
Based on my layman's understanding, quantum mechanics allows for energy to spontaneously appear, provided it disappears later. But matter, with its positive energy, is offset by the negative energy gravitational field it creates -- netting zero energy that therefore doesn't have to disappear.
Does this mean that matter/antimatter (say, an electron and a positron) may be created and annihilate each other, or maybe some amount of matter on its own (with gravity and thus a net energy of zero) could appear and exist forever? In the latter case, how does a small amount of matter compare with this supercompact form that leads to inflation? Presumably there are isolated hunks of matter with corresponding gravity out there that quantum mechanics created but that didn't expand into their own universes?