I purchased this Toshiba Satellite A500 about a week and a bit ago and this mark has appeared on the screen.
Is it dead/stuck pixels or is it cracked within? I haven't done anything to crack the screen so how could something like this appear?
That looks more like a crack rather than dead pixels.
If you run your finger over it, can you feel any bump? (May want to put a thin cloth there first so you don't leave any marks).
Anyway, no matter what, if you only bought it a week ago, they should have no problem in swapping it for a new one under warranty as if you did nothing to cause it*, it must be a manufacturing problem.
*e.g. no equal marks / dents on the lid in the same place.