As the universe expands it cools. Thus at early times, the temperature of the universe was much hotter. At sufficiently early times, the temperatures were sufficiently hot that particles moved fast enough to create particle-antiparticle pairs. As one goes back further in time, more massive particles could exist and at a higher density.
Times refer to time after the big bang .
The instant of the big bang refers to the instantaneous creation of all matter and energy from a single point in space-time. Reaonable observational evidence of the big bang only extends back to item number 2 in the list below. Most of the particles then represent pure energy, and few are from the restraints of baryon and lepton number conservation. At high temperatures, fundamental symmetries are restored, and the unification of the weak, electromagnetic, and possibly QCD and gravity are realized. Cooling has allowed nuclei to become bound.