system
Given the following post, respond as if you're commenting on Reddit:
human
Prior to the moment of aggressive inflation of the Universe, all matter was compressed into a space smaller than a proton. I understand the hypotheses leading to an expanding Universe, but why didn't all matter in such a tiny space simply result in a Universe-massed black hole?
gpt
1) most of the matter in the Universe was created well after inflation, and most of the energy was presumably created *by* inflation during the inflationary era, and the vast majority of it in the very last part. So it wasn't there in the beginning.

2) apart from this, a key hypothesis for the criterion that a sufficiently compressed mass makes a black hole is that if you move sufficiently far away from the mass spacetime becomes empty and flat (or, relatively emptier and flatter). So the black hole can then be a sac of spacetime which is "cut off" from a "normal" rest. In the very early Universe, this hypothesis fails because the Universe filled with uniform energy density is all there is, you cannot go away from it. It's impossible for a piece of it to be "cut off" from the rest because there's nothing special about any place in the Universe, at each time it's the same everywhere (that's the Copernican principle aka homogeneity).

3) minor point and mostly for fun... even if you relax homogeneity, the Universe in the inflationary era is exponentially expanding, or in another name it's a de Sitter spacetime. This shape of spacetime has a maximum size of black hole it can host, the faster the expansion the smaller the maximum BH. In the case of the inflationary era the upper bound is minuscule, and no significantly-lived BH can form.

3bis) during the later stages of the history of the Universe, where the acceleration rate dropped enormously in magnitude and became negative (so that the expansion slowed down) it's possible for inhomogeneities to have made black holes. These would be called primordial black holes and would be experimentally distinguishable from astrophysical BHs because they could get quite smaller than what a BH formed in stellar collapse can. They might have been small enough at formation to be finishing evaporating through Hawking radiation right now, so there are ongoing searches for the death signature of this hypothetical population.