The big bang starts off with a state of extremely high density...

The big bang starts off with a state of extremely high density and pressure for the Universe. Under those conditions, the Universe is dominated by radiation. This means that the majority of the energy is in the form of photons and other massless or nearly massless particles (like neutrinos) that move at near the speed of light. As the big bang evolves in time, the temperature drops rapidly as the Universe expands and the average velocity of particles decreases.
Finally, one reaches a state where the energy of the Universe is primarily contained in non-relativistic matter (matter sufficiently massive that its average velocity is very much less than the speed of light). This is called a matter dominated universe. The early Universe was radiation dominated, but the present Universe is matter dominated. Let us now give a brief description of the most important events in the big bang.
The Cast of Characters for the Big Bang

The primary cast of characters includes:

1. Photons ("particles" of light)
2. Protons and neutrons
3. Electrons and their antiparticles the positrons
4. Neutrinos and their antiparticles the antineutrinos

Look at the first sentence:
a state of extremely high density and pressure Where is this ? Where does this take place, where did the cast of characters for the BB come from ? Where are they as nothing exists ?