Every once in a rare while I get a "shower" of them, which is pretty disconcerting, but there's never been an underlying cause that my ophthalmologist could determine.
Those "showers" are likely red blood cells (which can be identified by their being black/dark with a white/lighter center, since RBCs are biconcave) and are likely due to a brief rupture of a capillary — and they dissipate almost as quickly as they appear.
