It all started with a singularity of infinite mass and infinite temperature. Suddenly and rapidly 13.8 billion years ago, the matter ballooned out of the singularity. Almost immediately time, space and physics formed. Soon subatomic particles like quarks begin to form. As the universe cooled, these quarks built neutrons and protons. Those protons and neutrons came together to form atoms once the universe cooled even more. The atoms formed only ever got as far a becoming hydrogen and helium before the universe had cooled to much to continue fusing them together. All the radiation from these events can still be measured by looking at the cosmic microwave background.
The universe is still expanding after this event. While originally it was thought to be slowing due to gravity produced by dark matter, it is now known to be accelerating, probably due to dark energy. Most of our knowledge on how the Big Bang may have happened and how it influences the universe today comes from our mathematical theory of surrounding principles and a minute number of observations. Today, it is the most commonly accepted theory on the formation of the universe.
There is more work being done to try and find more answers about the Big Bang as a whole, as well as the expansion of the universe. The Planck mission mapped the expanding of the distant universe to calculate the Hubble constant describing the rate, which varied from the Hubble Telescope’s near universe estimate. This means the universe is expanding at a different rate near Earth than what it is further from Earth. The European Space Agency has scheduled Euclid to launch in 2023 and has equipped it to further observe the universe’s expansion by observing dark energy.
References
Howell, E. & A. May. (Jan 2022). “What is the Big Bang Theory?” Space. Retrieved Jun 3, 2022, from https://www.space.com/25126-big-bang-theory.html
Institute of Physics. (n.d.). “The Big Bang.” Institute of Physics. Retrieved Jun 3, 2022, from https://www.iop.org/explore-physics/physics-stepping-stones/big-bang#gref
Wall, M. & T. Pultarova. (Feb 2022). “The Big Bang: What really happened at our universe’s birth?” Space. Retrieved Jun 3, 2022, from https://www.space.com/13347-big-bang-origins-universe-birth.html
Williams, M. (Dec 2015). “What is the Big Bang Theory?” Phys.org. Retrieved Jun 3, 2022, from https://phys.org/news/2015-12-big-theory.html
Wood, C. (Jun 2019). “The Big Bang Theory: How the Universe Began.” LiveScience. Retrieved Jun 3, 2022, from https://www.livescience.com/65700-big-bang-theory.html
Mysteries of Space 