Hum,
Well at least you grasped the idea proposed by the classic big bang theory…
Indeed, cosmologist George Gamow once in a conversation with Albert Einstein casually mentioned that one of his colleagues had pointed out to him that according to Einstein's equations a star could be created out of nothing at all, because its negative gravitational energy precisely cancels out its positive mass energy.
"Einstein stopped in his tracks, and, since we were crossing a street, several cars had to stop to avoid running us down".
There are a few experiments (i.e. COBE satellite) we can do to check that the universe also hasn’t a rotation, or that the universe is electrically neutral, and we can show that certain particle collisions [ as K-mesons, or kaons decay violated the so-called charge-parity CP symmetry ] have `chirality` (preferred direction) that show that the universe isn’t symmetrical at the quantum level, with respect to matter and antimatter.
There was a discrepancy in the decay between kaons and anti-kaons.
It’s worth tracking down a copy of the 1967 landmark paper by Andrei Sakharov; "Violation of CP Invariance, C Asymmetry, and Baryon Asymmetry of the Universe"
When the universe was creating matter and antimatter there was a slight imbalance (not a 100% annihilation process) that was due to the way that the original 10 dimensional symmetry broke down. The matter (the imbalance) that was left, was balanced by the creation of space-time.
One could postulate that a universe could produce matter and antimatter that cancelled out completely (i.e. no particles in it) but by doing so there is no creation of space/time/gravity…
Alternatively, that there was too much Baryonic matter created, and the universe quickly imploded, through gravity, before it could `inflate` (through inflation).
Or, slightly too little matter created, and the inflation energy is too powerful, and disperses everything so that stars and galaxies can’t form...
And indeed that is what quantum fluctuations may have done; there may have been an infinite amount of `failed` universes, that have occurred `before` our universe came about.
Quantum uncertainty allows the temporary creation of bubbles of energy, or pairs of particles (such as electron-positron pairs mentiond before) out of nothing, provided that they disappear in a short time.
(No need for a "force, necessarily being external to the jar")
The more mass created, the shorter the virtual particle can exist.
The energy in a (space-time) gravitational field is negative, while the energy locked up in matter is positive.
If the Universe is exactly flat, then the two numbers cancel out, and the overall energy of the Universe is precisely zero.