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.
Jonathan is a graphic artist, illustrator, writer, and game designer. Jonathan retired from the U.S. Army in 2017 and enjoys researching and writing about history, science, theology, and many other subjects. He writes for ScreenRant, CBR, NerdBastards, Listverse, Ranker, WhatCulture, and many other sites online. You can check out his latest on Twitter: @TalkingBull or on his blog: jonathanhkantor.com