This article contains spoilers for Fantastic Four #43.
The Fantastic Four have finally discovered the heart of Marvel's multiverse. Multiverses are all the rage nowadays, whether in the MCU or Buffy the Vampire Slayer. The comics themselves, of course, have been telling multiversal stories for decades; Marvel is known for its «What If?» stories set in alternate timelines, while DC has long enjoyed telling «Elseworlds» adventures set in very different realities.
It's long been clear there's a secret structure to Marvel's multiverse. The main comics timeline is consistently referred to as Earth-616; anyone who discovered the multiverse would naturally begin from their own frame of reference unless there was a structure, meaning there wouldn't be a common numbering system. That structure has, however, remained something of a mystery; there have been hints it is provided by nexuses of reality such as the Shi'ar M'Kraan crystal or the portals guarded by the Man-Thing, but these have never really been explained.
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Dan Slott, Rachael Stott and Andrea DiVito's Fantastic Four #43 finally reveals the very heart of Marvel's multiverse. According to the Watcher, above every nexus is the Apex of All Reality. This is found on one world in the 616 universe, and the Watcher's race dedicated themselves to protecting it, claiming the planet as their home. The Apex of All Reality is the secret of the Watchers' power, the key to their ability to monitor every timeline in the multiverse.
Fantastic Four #43 doesn't provide any more information about the Apex of All Reality; still, it's reasonable to assume it serves as the core of the multiverse, a convergence
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