Lots of particles created, now as numerous as photons. This era wound up determining that we wound up in a matter universe rather than an anti-matter universe . Same with electrons and neutrons. Why we see very little anti-matter now. Bonds break apart, loose protons and neutrons try to fuse again.
Era ends when temperature drops to about a billion degrees, and fused particles can stay fused.
It won't change until it starts being processed in stars. Hydrogen and helium nuclei all over, and free electrons. Very similar to the hot ionized regions around supernovae or hot young stars, or in the very center of stars. Photons do a random walk, as they do in the center of stars, and don't travel very far without running into an electron. When the universe cools to 3000 K, electrons can stay bound to the nuclei, photons can suddenly travel freely.