early universe - What was the entity/particle/state at the time of the Big bang?

The notions of today's universe don't make much sense for the (hypothetical) pre-Planck epoch of the universe. All four known fundamental forces should have been united. Matter didn't exist. It can form only in cooler conditions. Even cause and effect don't work well below Planck scales.

The structure you eventually get can be treated in terms of a groupoid, as described e.g. in this paper. Groupoids are related to groups (algebra). Groups actually are - in some sense - very simple groupoids. You may construct some groupoids from disjoint groups by connecting them via isomorphisms. So you get kind of a network of abstract entities.

Now try to adjust quantum mechanics and General Relativity, such that it fits with these structures, meaning without the smooth space and time, as usually assumed for standard physics. Let known or extrapolated physics of the Planck- and GUT (grand unified theory) epoch emerge from the pre-Planck physics to start the actual Big Bang.

It's possible to construct physical theories for a pre-Planck era. But it's very hard to find observational or experimental evidence to test these theories with respect to actual physics.

Those pre-Planck groupoid theories can be defined in a way that they don't need a predecessor epoch, e.g. by eliminating the macroscopic cause and effect paradigm, which is based on a notion of an arrow of time.
Causality and a concept of before/after on isomorphic entities doesn't make much sense.

Although, there should be some mechanism which eventually disrupted a strictly non-local pre-Planck configuration.