For the first few hundred thousand years after the Big Bang, the universe was a hot plasma in which photons scattered ceaselessly off free electrons. The cosmos was opaque: light existed, but could go nowhere. When the temperature fell to roughly 3,000 K, electrons combined with protons to form the first neutral hydrogen and helium atoms. The event is called recombination — a technical courtesy of language, since this was in fact the first time stable atoms had ever existed.

Once neutral atoms formed, photons no longer had free charges to scatter from; light abruptly began to travel in straight lines across the universe. This moment, the "surface of last scattering," is the earliest epoch we can directly observe: anything older is hidden behind opaque plasma. The light released then, stretched by cosmic expansion into the microwave band, is what we now measure as the cosmic microwave background — arriving from every direction in the sky.

The hundreds of millions of years between recombination and the first stars are known as the Cosmic Dark Ages: no source of light yet existed, only the slowly cooling background radiation. The first seeds of structure — faint temperature ripples imprinted on the CMB — were quietly growing in that silence into what would eventually become galaxies.