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.
(..) Tommy! Why can't I wear your stuff? Donna, not you! How did I get in my house? How did I get in my bed? Life is full of mysteries, Donna. Are you mad at me for wearing something of yours? I don't want you to wear my stuff. I do love you, Laura. (..)1992 Twin Peaks: Fire Walk with Me