Why was the universe opaque before 380,000 years of its birth?
Prior to “recombination”, when the universe became cool enough for electrons and nuclei to get together and form atoms, the universe was made of plasma, and lots of free electrons were floating around. Because photons interact much more readily with free electrons than with electrons bound in atoms, the universe was pretty much opaque: photons never got very far (in the same way that the Sun, which is also mostly made of plasma, is opaque). With electrons finally safely nestled in atoms at 380,000 years, photons were free to stream around and the universe became transparent.
