10-3 second to 3 minutes

At last we’ve reached a time — the Era of Nucleosynthesis — that we can really begin to wrap our heads around.

For reasons no one yet fully understands, antimatter has now become exceedingly rare. As a result, annihilations of matter and antimatter no longer happen as often. This allows our universe to grow almost entirely from that leftover matter. Space continues to stretch, too. The energy from the Big Bang keeps cooling off, and that lets heavier particles — like protons, neutrons, and electrons — begin to form. There’s still lots of energy all around, but the “stuff” of the cosmos has stabilized so that it is now almost entirely made of matter.

Protons, neutrons, electrons and neutrinos have become abundant and are beginning to interact. Some protons and neutrons fuse into the first atomic nuclei. Still, only the very simplest ones can form: hydrogen (1 proton + 1 neutron) and helium (2 protons + 2 neutrons).

By the end of the first three minutes, the universe has cooled so much that this primordial nuclear fusion comes to an end. It is still too hot to form balanced atoms (meaning, with positive nuclei and negative electrons). But these nuclei seal the makeup of our cosmos’ future matter: three parts hydrogen to one part helium. That ratio is still much the same today.