Leaving the highly speculative first second, and skipping to a few minutes later, the Nucleosynthesis period accurred about 3 to 20 minutes after the Big Bang. During this period, the universe had cooled enough for protons and neutrons to come together to form simple nuclei, but not yet atoms, as the universe remained too hot for electrons to bind to these nuclei. The nuclei formed during this time were primarily those that would become the core of hydrogen atoms (including its isotope deuterium), helium, and trace amounts of lithium, through a process known as Big Bang nucleosynthesis.

Supported by empirical data. The observed abundances of light elements in the universe align closely with predictions from Big Bang nucleosynthesis models, providing strong evidence for this period of early cosmic history.