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.
(..) And the woman was arrayed in purple and scarlet color, having a golden cup in her hand. And upon her forehad was a name, written, mystery, Babylon the Great, the mother of abominations of the Earth." "You will destroy Joh Fredersen - him and his city and his son -!" Gluttony, Greed, Pride (..)1927 Metropolis