Or the connectivity between universes is sparse, so that we only have direct connections to a few universes. I guess the set of all universes might still be a closed system in this arrangement.
This is overall kind of an interesting idea, because it seems to me like it'd be more testable than some other more intangible multiverses. My (non-physicist) intuition has always been that black holes somehow are the key to many mysteries of physics, but we're still pretty far from really knowing I think.
Riffing off that a bit, I think back to this illustration of spacetime - https://simple.wikipedia.org/wiki/Space-time#/media/File:GPB... - and imagine a black hole being a place where that fabric is punched through. as if spacetime is just resting on another universe. Where things get massive enough, they punch through and the matter begins to drain away. We don't know what the rate of flow is, though.
I like this because referring back to the comment about not being able to see enough matter - we know black holes aren't massive enough to account for all the dark matter - but, what if the discrepancy of observation vs expectation gave us some idea of what the flow rate through black holes might be?
This is overall kind of an interesting idea, because it seems to me like it'd be more testable than some other more intangible multiverses. My (non-physicist) intuition has always been that black holes somehow are the key to many mysteries of physics, but we're still pretty far from really knowing I think.