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.
(..) was a happy man who lived life to the full. He loved his family and his work, and that he should have taken his own life in this way is a mystery and a shock to all who knew him. Yes, yes! Taxi! - I'll be back in two minutes, mate. - What? - I'm just going home to get my umbrella. (..)2010 TvShow: Sherlock Title: A Study in Pink Season: 1 Episode 1