So, during the first few minutes after the starting point, the universe was
hot enough to fuse particles into helium nuclei. The result was a ratio of about
12 hydrogen nuclei to 1 helium nucleus; that's equivalent to saying that three
quarters of the mass of the universe was hydrogen nuclei, and one quarter of the
mass was helium nuclei.
But these nuclei were totally ionized: they lacked the normal
collection of electrons surrounding them. The electrons were free to fly around
space on their own. Free electrons are very efficient at scattering photons. Any
light rays or radio waves or X-rays in this ionized plasma were scattered before
they could travel far. The universe was opaque.
After a few thousand years, as the universe continued to expand and cool, the
temperature reached a critical point. About 100,000 years after the starting
point, the temperature dropped to about 3,000 degrees Kelvin. At this point,
hydrogen nuclei (protons) were able to capture electrons, and hold them against
collisions. We call this process of capturing electrons recombination
(even though it was really the first "combination", not a re-"combination").
The universe became largely neutral, with electrons bound into
hydrogen and helium atoms. Neutral atoms are nearly transparent to light rays
and radio waves. Suddenly, the universe became transparent.
