The Big Bang Theory

image from NASA.gov illustrating the different stages of the evolution of the universe according to the Big Bang Model

The most readily accepted model, for the formation of the universe is the Big Bang Model. It states that everything contained within our universe  must have originated at a single point of infinite density and massive heat around 13.8 billion years ago, that then began to expand. After the first stages of expansion, the young universe cooled down enough for the formation of subatomic particles and then atoms. Massive clouds of these atoms then coalesced through the gravitational force to form stars and galaxies.

The earliest period of the universe is known as the Planck epoch. When all the matter was compressed into a single point of infinite density and massive heat. During this Planck era it is believed that the quantum gravity dominated physical interactions. The Planck epoch extends form the start of the universe until econds. From the universe cooled enough to cross transition temperatures, At this time the fundamental forces of the universe started to separate from each other, the gravitational force began to separate from the strong nuclear, weak nuclear and electromagnetic forces. Then from 10^{-36} to 10^{-32} seconds, the electromagnetic and weak nuclear forces separated.

Inflation Epoch

The inflation epoch started when the first fundamental forces formed. This epoch began at  10^-32 seconds and was characterised by high energy density, extreme temperatures and pressures, which gave rise to rapid expansion and cooling.  The temperatures were so high that the random motion of particles occurred at such high speeds that particle-antiparticle pairs kept being created and destroyed by collisions which may have led to the dominance of matter over antimatter in our universe. After inflation ended the universe was composed of a quark-gluon plasma , after this point, the universe began to cool matter began to coalesce and form.

Cooling Epoch

The universe continued on decreasing in temperature and density. After a few minutes, temperatures dropped to 1 billion Kelvin causing protons and neutrons to form the universes first deuterium and Helium atoms.

After 379,000 years radiation decoupled from matter and expanded through space, this radiation is known as the Cosmic Microwave background and it is the oldest light in the universe.

Structure Epoch

For the next several billion years, the slightly denser regions of the universe started to become gravitationally attracted to each other. These grew even denser and formed gas clouds, stars, galaxies. The baryonic matter that makes up for all the stars, gas clouds and other visible sources of matter only account for 4.6% of the matter in the universe. With the rest being composed of dark matter , which we know very little about.

References

https://phys.org/news/2015-12-big-theory.html