What does it mean if "The universe was opaque to radiation" at one time? The time period to which this is referring is during the epoch of decoupling, if you know what that is. (During the formation of atoms in our universe) What do astronomers mean by "The universe was opaque to radiation" and, also, by saying that, at early times matter was ionized? And that, the universe was filled with free electrons? And that matter was ionized? By the way what does it mean for matter to be ionized? Does this just mean that, the matter's configuration was in the standard model (protons, nucleus, and electrons) and that the matter had too many electrons, which made it ionized? or that it was a cation and an anion?