When a superhero comes across a mystery in a comic book, it usually becomes the focus of their adventure, and by the end of the book (or arc), that mystery is solved. That makes sense, and it's pretty much how storytelling works, but every so often, a mystery comes along that even Batman himself has trouble solving.
Granted, it's rare, and there's a good reason for that. Batman wouldn't be the World's Greatest Detective if he got stumped every other issue. Even though he's good at his job, he's dropped the ball on sleuthing a few things over the years, and he's not the only one.
Over in Marvel, Image, and numerous other universes, there are superheroes who have had to scratch their heads and admit there are just some mysteries that can't be solved by the end of a comic book. When it happens, the fans tend to speculate, but even the Internet can't answer everything.
There actually aren't all that many mysteries superheroes had trouble solving given the length of time their books have been out, but there are at least ten. The ones featured here are either the hardest ones to solve, or they remain a mystery because the writers and artists simply haven't given anyone the answer. At least not yet...