|UGC 12158, photo by Hubble/NASA|
In the first seconds after the Big Bang, radiation dominated, then matter dominated with formation of galaxies.
Electrons and positrons collided, annihilated one another, and created gamma ray photons. For a brief time, there existed an equilibrium between matter, anti-matter, and radiation. However, there was an excess of 1 matter particle out of every billion particles, so the system was no longer in equilibrium, although this process is not yet well understood.
During the first few minutes, the universe expanded and cooled; protons and neutrons formed a few helium nuclei, the rest of the protons remained as hydrogen nuclei. Free neutrons have a half-life of 15 minutes, so if they get locked up in helium, they're are out of circulation; others wound up decaying to produce more hydrogen. Some of these processes extended beyond first few minutes, but helium is a limiting factor, tying up neutrons.
The radiation dominated era produced hot plasma (ionized gas) and a period of expansion and cooling, but no new synthesis of elements. Photons were incessantly scattered by free electrons, in equilibrium with matter.
Then followed an era of recombination. Protons and electrons recombined to form atoms, in a transition to the matter-dominant era. Temperature fell to a few thousand K, like the surface of a star. Gas was no longer ionized plasma, but neutral hydrogen, which does not react so readily to photons, so the universe became "transparent" to photons. Radiation could penetrate billions of light years and be detected by us. This is where microwave background radiation comes from.
Reionization. After less than 1 billion years, the first stars formed; ultraviolet radiation from the first stars and quasars (emit ionizing radiation) re-ionized the gas in the early universe. Bubbles of ionized gas overlapped, and the universe became opaque again.
Cosmological principle assume: Homogeneity, that the local universe has same physical properties throughout the larger universe, each region has same physical properties (mass density, expansion rate, visible vs. dark matter). Isotropy (no preferred direction); on largest scale, local universe looks the same in any direction. Universality; the laws of physics are the same everywhere in the universe (size scale > 100 million parsecs.)
Space itself seems to posses a "dark energy" or anti-gravity that acts to expand space, an intrinsic energy associated with space/vacuum. We have no clue what this is, but it seems to be the most important energy constituent of the universe today.
Until 6 billion years ago, the gravitational energy of matter was stronger than the acceleration forces, but then dark energy began to dominate energy density of universe. We see an anti-gravity effect that pushes things apart faster and faster. Today, the acceleration due to dark energy dominates, creating an exponential expansion. If the energy density remains constant, we get a future "the big empty." Galaxies will move away faster than the speed of light, so their light will never reach us. Within the Milky Way, however, systems will be held together by gravity, but we won't be able to see beyond our galaxy.