Before the decoupling,

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Before the decoupling, the universe was filled with a hot, dense plasma of photons, electrons, and baryons (protons and neutrons). This plasma was opaque to radiation because photons were constantly being scattered by the free electrons in a process known as Thomson scattering. It wasn't until the universe cooled enough for electrons and protons to combine and form neutral hydrogen atoms—a process known as recombination—that the universe became transparent to radiation, allowing photons to travel freely. This marked the era of decoupling.

Before the Decoupling