What happened immediately after the Big Bang occurred?

Right after the big bang the universe expanded in a millionth of a second to half the size it is today. (This, by the way, defies the universally accepted axiom that nothing can travel faster than the speed of light when it is quite obvious the first expansion of the universe did exactly that.)

After the Big Bang, the universe expanded quickly and then started to cool down. Elementary particles, like protons, neutrons, and electrons, as well as their antiparticles, formed. As the universe cooled down more, these particles joined to form the nuclei of hydrogen and helium. After the universe cooled down enough, neutral atoms formed, allowing light to travel freely. This era of recombination is the earliest period from which light can be observed, and it is also known as the cosmic microwave background radiation.