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Why was the universe opaque to light before atoms formed?

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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.

Before the formation of neutral atoms, the early universe was filled with ionized matter, which rendered it opaque to electromagnetic radiation. In this state, free electrons frequently scattered photons, preventing light from traveling freely through space.1

As the universe expanded and cooled, it eventually reached a temperature low enough to allow for the formation of neutral atoms. Once these atoms formed, the free electrons were bound, and the universe suddenly became transparent to radiation, much like how light travels through air today. This transition is known as the "decoupling time," at which point the radiation began to travel unimpeded through the cosmos. Over billions of years of expansion, these original high-energy photons have been stretched into the microwave wavelengths we observe today as the cosmic microwave background.12