Explain how hydrogen and helium nuclei were first formed in the universe.

Answer:

Hydrogen and helium nuclei were formed in the universe through a process called nucleosynthesis. As the universe cooled after the Big Bang, protons and neutrons combined to form deuterium, which is an isotope of hydrogen. Neutrons then combined with deuterium nuclei to form tritium. Finally, a proton fused with a tritium nucleus to form a helium nucleus.