Big Bang is an expansion of universe, that is thought to start from a singular point according to Einsteins general theory of relativity inspit of having complexity on singularity conception of physics.
How was our universe at the initial state of Big Bang? The universe was a hot and dense state. Once the hot and dense state started to expanse.
It is also thought that after few minute of Big Bang matter started to form. After few minute formed lighter element. It is thought that the heavier elements had to taken more times to form.
The abundance of lighter elements in universe is another verification of Big Bang theory.
BBFH theory says that all of the elements of universe were produced in stellar during supernovae period.
BBFH theory have some limitation. Because, BBFH theory can not adequately explain the cause of abundance of helium and deuterium in universe. Stellar theory says that deuterium can not produce in stellar interiors. Exactly, Deuterium will destroy inside the star.
But in universe there have abundance of deuterium. But, how deuterium can be produce?
After big bang when the temperature was rapidly cooling and a favorable temperature achieved that favors Nucleosynthesis. Protons and neutrons collides to produced deuterium.
Big Bang Nucleosynthesis predicts the abundance of helium in universe, deuterium and a smaller amount of lithium else.