https://ned.ipac.caltech.edu/level5/March10/Garrett/Garrett3...
What would they have been made up? Do quarks have mass, and there was enough of them? Or does energy through E=MC^2 mean that enough pure energy density can cause a black hole?
Yes! https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)
I'm not sure exactly when primordial blackholes would have formed but I think it would have been sometime after inflation, though perhaps before baryogenesis. Here's the reasoning: I think it would be after the GUT epoch so gravity would have splintered off from the other forces and I think it would have been after inflation started so that inflation had a chance to exaggerate the scale of quantum fluctuations to create the necessary size & scale of density fluctuations for blackholes to form.
Energy alone is enough to form a blackhole. However, it's not the energy density per se. That's sort of a necessary but not sufficient condition since you also need the surrounding spacetime to be at a low enough density relative to the region where you are expecting a blackhole to form.
This, incidentally is a common source of confusion about the big bang. "Why didn't it just form a blackhole?" The answer is because ALL of the spacetime was at the same density.
I could say that a small sphere is denser than a large sphere because the distances between points are smaller on average, but that requires the spheres to be embedded in something with a distance metric(Euclidean 3-dimensional space).
What is the thing that spacetime is embedded in that provides a pointwise distance metric?
A technical explanation of the expansion involves the inflaton field.
(There's also energy density of the gravitational field.)