Question

Why is the universe transparent?

Answer 1

The universe was opaque before the recombination, due to the scattering of photons (of all wavelengths) off free electrons (and, to a significantly lesser extent, free protons), but it became increasingly transparent as more electrons and protons combined to form neutral hydrogen atoms. While the electrons of neutral hydrogen can absorb photons of some wavelengths by rising to an excited state, a universe full of neutral hydrogen will be relatively opaque only at those absorbed wavelengths, but transparent throughout most of the spectrum.
<Wikipedia>