The other day, I was on Wikipedia trying to look up the name of those sparkles that happen when you sneeze too hard (which happen to be phosphenes, by the way).  Then the magic of Wikipedia links happened, and check this out.  Eigengrau.  Also known as brain gray, it’s a shade of dark gray that the brain perceives when no light is entering the eyes.  Your nerve cells still fire at a baseline level even when they’re receiving no input.  Cool, huh?