Who can tell?
No one really knows (to truth about or answer to something). A: "What does the future hold, now that you've left the music industry?" B: "Who can tell, really? I've always been interested in writing a novel, so I may pursue that." A: "Why on earth have they decided to go this route with the new software?" B: "Who can tell what convoluted analytics they employ to come up with their new designs?"