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.
(..) But so much more interesting. What makes your husband a bad man? His nature, I suppose. The nature of his work? A mystery, I'm afraid. I'm also afraid you will sleep with me in order to get to him. How afraid? Not enough to stop. Can I ask you a personal question? (..)2006 Casino Royale