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.
(..) Imagine if the time machine fell into the wrong hands? DOC: My only regret... But time traveling is just too dangerous. Better that I devote myself to study... ...the other great mystery of the universe: Women. DOC: Marty, Einie, brace yourselves for temporal displacement. Did we make it? (..)1989 Back to the Future Part II