There's no 'mystery' (to/as to/about) why we're here. = It's no 'mystery' why we're here. [=we know why we are here]
His success is something of a 'mystery'. = His success is a bit of a 'mystery'.
We'll reveal the identity of today's 'mystery' guest after these commercials.
(..) Well, by no stretch of my imagination do I believe you've all come here to hear me lecture. Or rather, to ascertain the identity of the mystery math magician. So without further adieu, come forward, silent rogue, and receive thy prize. Well, I'm sorry to disappoint my spectators, but... (..)1997 Good Will Hunting