The Big Bang theory is a widely accepted explanation for how the universe originated and evolved. According to the Big Bang theory, the universe was extremely hot and dense when it was born 13.7 billion years ago. The rapid expansion, called inflation, is thought to have occurred at a rate faster than the speed of light.
Before the Big Bang, space didn’t exist. It’s likely there was an area of matter squeezed into a small, hot dense area. Scientists dubbed this a singularity.
The Big Bang suggests our universe was very hot. Astronomers discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation in the observable universe that shows some remnant of this heat.
Our universe was so hot that it took thousands of years for matter like protons and neutrons to cool down enough for atoms to form. And it took about 400 million more years before matter came together to form clumps of matter that developed into stars and galaxies. Our own solar system wasn’t born till about 9 billion years after the Big Bang began. [7 Surprising Things About the Universe]
One common misconception is the Big Bang was a major explosion. Since there was no space before the universe began to expand, theory holds that matter began to simply form and expand all at once in the darkness.
In the 1920s, Astronomer Edwin Hubble realized the universe was still expanding and it was doing so at a very fast rate. Scientists say the cause of this acceleration could be related to dark energy in our universe. How dark energy works in our universe remains one of the greatest mysteries for scientists.