When the universe was younger than about 300,000 years, the temperature was high enough that all of the hydrogen was ionized, that is the electrons were free and separate from the protons. Because of the presence of the free electrons, photons were scattered around in all directions and could not travel far before changing their direction. Therefore the universe was "opaque".
When the universe cooled to about 3000 K, the electrons and protons combined to form hydrogren atoms. After "recombination", photons were able to travel through the universe relatively unimpeded, and the universe became "transparent".
The photons emitted when atoms formed comprise the Primordial Background Radiation, the 3 degree blackbody radiation detected by Penzias and Wilson in 1965.
Before recombination, all of the electrons were free.
After recombination, the electrons and protons formed atoms.
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