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How did neutral atoms make the early universe transparent to electromagnetic radiation?

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Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

In the early stages of the universe, matter existed in a highly ionized state, meaning electrons were stripped from nuclei. In this environment, free electrons frequently scattered electromagnetic radiation, rendering the universe opaque to light.1

As the universe expanded and cooled, it eventually reached a temperature low enough to allow for the formation of neutral atoms through the process of recombination. Once electrons were bound to nuclei to form these neutral atoms, they were no longer free to scatter photons. This transition, known as the "decoupling time," suddenly allowed electromagnetic radiation to travel freely through space, making the universe transparent. This "first light" from the era of recombination has since been redshifted by the continued expansion of the universe, becoming the cosmic microwave background observed today.12