What is the Dark Mark that goes on the Death Eaters' arms?
How is it put on their forearms?
How painful is the application of the Dark Mark?
Who gets the Dark Mark applied?
What kind of protection is on it so not just any Auror can pull up a Death Eater's sleeve and see the mark?
What happened to the Dark Mark during the Vapormort years (after Harry "defeated" him and before he was resurrected in the graveyard)?
Any other personal theories concerning the Dark Mark?