During this enormously long Era of Atoms, matter grew into the remarkable variety we now know. The stable atoms of hydrogen and helium slowly drifted together in patches, due to gravity. This further emptied space. And wherever the atoms clumped, they heated up.This was a dark time for the universe. Matter and space had separated from each other. Light could travel freely — there just wasn’t much of it. As clumps of atoms grew both bigger and hotter, they eventually would start to spark fusion. It’s the same process that happened before (fusing hydrogen nuclei into helium). But now the fusion wasn’t happening everywhere, evenly. Instead, it became concentrated in the newly-forming centers of stars. Baby stars fused hydrogen into helium — then (over time) into lithium, and later still into the much heavier elements such as carbon.

Those stars would generate more light.

Throughout this Era of Atoms, stars began to fuse hydrogen and helium into carbon, nitrogen, oxygen and the other light elements. As stars grew older, they became able to exist with more mass. This, in turn, spawned heavier elements. Eventually, stars were able to burst beyond their previous bounds into supernovas.

Stars also began to attract one another into clusters. Planets and solar systems formed. This gave way to the evolution of galaxies.