Building the First Atoms

For the first 380,000 years, the universe was an opaque fog of plasma — free protons, neutrons, and electrons smashing into each other. Photons couldn't travel more than a short distance without being scattered.

Electrons finally slowed down enough to be captured by protons, forming neutral hydrogen and helium atoms. This event, called recombination, made the universe transparent for the first time. Photons were suddenly free to stream across space uninterrupted.