I think the multiverse is a simulation/experiment being run by the beyonders. The reason the beyonders can't find battleworld is because it's not on the hard drive, it's located on either the bios or the ram of the computer. That's what the white space is. I posted my full theory in this thread...
http://community.comicbookresources....-a-Simulation)
But, imo, the basic gist is...
A. All of reality is a simulation (an experiment as Hickman described it) run by higher beings (beings represented by the Beyonders inside the simulated reality). The beyonders are just representations of the beings, like computer code, similar to the mapmaker code trying to trace back the source of the incursions. The higher beings that the beyonders represent are inaccessible to anyone inside the simulation. By the way, if it's not clear, I'm using the components that a computer needs to function purely as an analogy. It's the easiest way I could come up with to describe the role each piece plays in Hickman's story.
B. The higher beings (manifested inside the simulation as the beyonders) are ending the simulation and the molecule man is our perception of the code that will terminate the simulation. Entities like The Living Tribunal and the ex nihilo serve important core functions in the BIOS that oversee the simulation and seed it with life. Galactus serves a role analogous to a hard drive defragger. Thus, as powerful as they seem, they are merely a part of the simulation and no longer serve a function now that the simulation is ending.
C. Doom found out 25 years ahead of the termination date and the incursions are the byproduct of him trying to save the simulation. His actions effectively recruit multitudes of intelligent entities in the simulation towards the task of preventing the simulation from being terminated.
D. Battleworld is analogous to a computer's boot OS built by doom by salvaging parts of various realities. It is hidden inside the BIOS aka the white space of the "computer (just an analogy)" running the simulation. It's likely that The Living Tribunal existed in the BIOS since The Living Tribunal is a security protocol for the entire simulation/multiverse. This is why The Living Tribunal's code is being used as a life raft to transport a handful of survivors over to the BIOS.
E. Once the Beyonders finish shutting down the original simulation and effectively reformat the hard drive, this boot OS (battleworld) will serve as the interface that allows Franklin Richards to recreate the multiverse from inside the BIOS itself.