Postpocelipse said:
This simulation suggests that the universe was massed into galaxy clusters immediately following BB or am I missing something?
I haven’t looked at the simulation, but galaxy clusters certainly did not exist immediately following the Big Bang: it was still far too hot for stars to form, and would remain so for several hundred million years.
The matter in the very early universe was an almost perfectly homogenous plasma consisting of approximately 75% hydrogen-1, 25% helium-4, and traces of other isotopes of hydrogen & helium, plus a little lithium.
Protons & electrons were formed when the universe was less than 10 seconds old. The hadrons (protons, antiprotons, neutrons, antineutrons) formed first, from t ~= 1 microsecond to t = 1 second. Most of these annihilated with each other (t = 1 second to t = 10 seconds), producing leptons (electrons, positrons, various flavours of neutrinos & antineutrinos).
After a couple of minutes, the universe had cooled enough for heavier nuclei to be stable, so from around t = 3 minutes to t = 20 minutes many of those remaining protons participated in nuclear fusion, creating the plasma I mentioned above. Of course, the fusion generated a lot of heat, but by 20 minutes the temperature had dropped too low for fusion to continue.
Note that the universe didn’t cool down by radiating the excess heat into the surrounding space: there is no surrounding space, and conditions throughout the whole universe were almost perfectly homogenous, apart from minute variations in density, and even more minute variations in composition. The cooling happened due to expansion.
The plasma was opaque (like a flame, or the gas in a modern street light), and it remained opaque until t = 380,000 years, when it was cool enough for the ions & electrons comprising the plasma to combine into atoms. The light that was released when the plasma condensed into gas and became transparent has now been shifted by expansion from the visible (& UV) part of the spectrum to the microwave part of the spectrum, and is now known as the Cosmic Microwave Background radiation (CMB or CMBR).
Then there was a long wait for this (almost) homogenous gas to condense due to gravitational attraction, leading to star formation and thence galaxy formation. As gas condenses, it tends to heat up, and if that heat has nowhere to go it tends to cause the gas to expand, which severely slowed down gravitational condensation in the early universe. (Even in the modern universe, where there’s plenty of open space to radiate heat into, the initial stages of star formation are slowed by that effect).
The earliest stars formed some time around t = 400 million years. The time between t = 380,000 years and t = 400 million years is known as the dark ages, as there were no sources of electromagnetic radiation at that time, apart from the CMB (which during the dark ages got shifted down to the infra-red).
It is believed that the gravitational collapse of the gas filling the universe amplified the minute variations in density mentioned earlier, so that those variations essentially acted as nuclei for star & galaxy formation.