OK, so the floor is open for guesses about what’s in it.
Personally, I think it’s Cecil’s fortress of solitude, where he goes when he’s not busy defeating mad scientists, thwarting alien invasions, and general saving of the world. Oh, and incidently, answering questions for the Teeming Millions.
The pyramids were all robbed shortly after they were built. The Great Pyramid, for example, was robbed by someone (well, a team of someones, realistically) who knew where the the Grand Gallery is and how to get around the anti-theft sliding blocks. If there’s more rooms in the pyramid, their construction wouldn’t have been a secret. I doubt it’s a burial chamber, and if it is, I doubt it’s undisturbed.
My money’s on either a) they used some shoddy blocks on the interior, which have crumbled under the weight of time or b) it’s a load distribution feature, like the false ceilings above the King’s Chamber.
Could it be just skimping on material? I mean, there doesn’t seem to be a terribly good reason to build the whole thing solid, and getting all those big stone blocks out and up there, however they did it, can’t have been very easy. So just leave a few gaps here and there, save a couple of big blocks, make the whole structure a little lighter, get done a bit quicker? (This sort of presumes that the muon scanning technique they used just resolves average density, IOW they merely know that there’s a volume where density is less than it would be for a completely solid structure.)