Only ghosts know why there is no information about them? Sorry I don' have dictionary on this phone so I can't check. The troll answers tho
How can we be called strong, different periods have different standards. However, one thing is sure, those "top gods" who are top-level supervisors may be able to drill out of the unpredictable universe somehow and know why he is unknown. -Google Translate
I'm assuming you only need the red? No one knows (you can also make it a question, as in "who knows?") why he's so mysterious. (lit. Ghosts knows why he's so mysterious/obscure, meaning is that only imaginary beings/no one can know.)
"who the hell knows why he was unknown" You can replace "hell" if you want but I felt it kept the meaning.