For the most part, a secret in a comic book isn't something that remains secret for long. Clark Kent's secret identity was only secret to only a few people (until recently), and most things a superhero or villain would prefer to keep under wraps is told immediately to the reader.
That's how the storytelling and exposition work in comics, but occasionally, a secret will be kept form readers for a time, only to be revealed later. When this happens, it's typically something like the identity of the Green Goblin, which was kept for two years.
Those kinds of secrets are often kept for the purpose of building wonder and suspense, but occasionally, a secret is created accidentally, or through no fault of the writers and artists. Those secrets are the ones that may not have been intentional due to the surprised popularity of someone like Wolverine, who's true identity wasn't revealed for decades.
Even rarer is the secret that's yet to be cracked. The ones that have been around for long periods of time tend to sit for years with the fans left wondering the truth. Those secrets are the hardest on the fans since there's no guarantee they will ever be revealed.
10. The Mystery Of The Three Jokers
In the DC Universe's comics, there's one mystery that has only gotten harder to uncover as time goes by, and it's all about the Joker. Canonically, the Joker's backstory and identity have never been revealed, though The Killing Joke did offer one possible origin story.
In 2015, Batman got a chance to sit in the Mobius Chair, and the first thing he asked was, "What's the Joker's true name?" It wasn't revealed right away, and the following year, Justice League #50 revealed that the answer wasn't simple; there were, in fact, three Jokers.
That revelation stunned the fandom, and Geoff Johns' telling of the Joker and his identity remained a mysterious point of contention among fans. Who were these three Jokers? Is it a metaphor, or multiple identities? There's no way of knowing, and for five years, the fans have been left wondering.
Theoretically, the answer to this mystery will finally be revealed on November 10, 2020. That's the release date for Johns and Jason Fabok's Batman: Three Jokers. The book is set to release three decades after The Killing Joke, and it will examine the who, what, and why of the Joker.