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.
(..) Only you. Thank you, good doctor. Now... this bomb is armed! And this bomb is mobile! And the identity of the triggerman is a mystery. For one of you... holds the detonator! Now, we come here not as conquerors... but as liberators, to return control of this city to the people. (..)2012 The Dark Knight Rises