No, I don’t know what the dark thing is, or the explanation; go poke around on NASA if you want to. [Update: Sam Ruby and someone else whose email I lost both tell me the dark thing is the planet’s shadow. Hmm. Sam also pointed to this.]
Also, look at some of the close-ups of the rings; not the pretty false-coloured versions as rendered by NASA’s picture-prettifiers, just the raw shots of endless horizontal bands of grey-on-grey over the star field. On average, one out of every 14 times you visit ongoing, that’s what you’ll see behind the title, above.