First nucleus of universe
http://yoitect.com/5hmq is the link to describe about Big Bang. Big Bang is nothing but an expansion. But it not an explosion.
If you read about big bang in http://yoitect.com/5hmq you will see that Nothing matter were exist before big bang. It was a hot and dense state.
After a few moment of Big Bang when the universe started to rapid cooling, it innitiates to form matter by converting energy into matter.
The first nucleus of universe formed after three minute of big bang.
What is Big Bang？
Is it an expansion or explosion？
Some people make a mistake thinking Big Bang as an explosion. But it is not true. Basically Big bang is an expansion of universe.
Once from a hot and dense state from a singular point, everything of the universe is started to expand. This expansion is called Big Bang.
Before Big Bang what was the feature of Universe. It may be a state of energy. Because there have no any matter in universe.
Acording to einsteins mass energy transformation equation, mass can transform into energy.
In that context, reversivly we can say energy also can convert into mass. Due to inter convertion of energy into mass there started to form of the universe.