How did the cosmic microwave background form when the early universe became transparent?

In the early stages of the big bang, the universe was filled with a hot, dense plasma of ionized particles. Because this plasma was opaque to electromagnetic radiation, light could not travel freely through space.12

As the universe expanded, it gradually cooled. Approximately 380,000 years after the big bang, the temperature dropped sufficiently to allow for the formation of neutral atoms—a process known as recombination. With the electrons captured into stable orbits around nuclei, the universe suddenly became transparent to radiation, allowing photons to travel unimpeded.312

This "first light," emitted at the time of decoupling, originally possessed very high energy and short wavelengths. Over the subsequent 13.8 billion years, the continued expansion of the universe has stretched these wavelengths, shifting them into the microwave region of the electromagnetic spectrum. Today, this residual radiation is observed as the cosmic microwave background, which provides a uniform, faint glow across the sky at a temperature of approximately 2.7 K.1345