The universe that we know today all started way back about 13.7 billion year ago.
In the beginning, there was nothing.
And then there was everything.
Our universe first started as an infinitely hot and infinitely dense point. Contrary to many people’s beliefs, the big bang was not an explosion. The hot and dense point started to expand more and more into eventually what our universe is today. In today’s world, everything is made out of atoms. An atom is made up of three tiny subatomic particles, which are protons, electrons and neutrons.
Minutes after the big bang, stable atoms were unable to form because of the extreme temperature. Atoms were unable to hold on most of their electrons because of the speed of particles moving around.
Seconds after the big bang, it so hot that protons and neutrons were unable to stay together because even when they formed, they would just smash into each other and break apart again.
Before even that, even quarks did not stay together. Quarks are basic particles, so they can’t be divided anymore.
Most of what we know about the beginning parts of the universe are results from experiments done in particle accelerators. Particle accelerators are machines that speed up subatomic particles to the point where they collide with each other which is exactly what happened after the big bang. The bigger particle accelerators that we are able to create, the further back we would be able to see.