History of the universe
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.
<10-12 sec: The universe was so hot, that the energy of the particles easily exceeded known masses. 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.
102-103 sec: This is the era of cosmic nucleosynthesis. Cooling has allowed nuclei to become bound. Light nuclei are formed such as helium and lithium.
1011 sec: Atoms form as electrons fall into their lowest orbitals. At this same time, most of the photons stop colliding and cool without collision. The remnants of this decoupling is the three degree background radiation.
1016 sec: Galaxies, stars and planets begin to form.
1018 sec: The present. The universe keeps expanding and Michigan State University flourishes.
1045 sec: Protons decay (perhaps) atomic matter ceases to exist.