Final answer:
Big Bang Nucleosynthesis occurred within a few hundred seconds of the Big Bang and resulted in the creation of helium and other elements.
Step-by-step explanation:
Big Bang Nucleosynthesis occurred within a few hundred seconds of the Big Bang. During this period, protons and neutrons combined to form deuterons, tritons, and helium nuclei. The process lasted about 3 minutes and resulted in the creation of approximately 25% of all matter in the form of helium, along with small percentages of other elements such as deuterium and tritium.