Complex sentence: The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis, which formed the hydrogen and helium that became the content of the first stars, and is responsible for the present hydrogen/helium ratio of the cosmos.
Split into simpler sentences: The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis. It was then that the hydrogen and helium that became the content of the first stars was formed, and is responsible for the present hydrogen/helium ratio of the cosmos.