The formation of the first atoms was the key event in cosmic history as far as observational cosmologists are concerned. Before the electrons and protons had become bound together into atoms, all of space was filled with a plasma of electrically charged particles—a plasma that was almost entirely opaque to light. For this reason, our telescopes can observe only the light that originated after this key event. Using a telescope to look farther back in time is as difficult as using one to look deep beneath the surface of the Sun.
