Full Idea
Any other realms that happened to exist would just be scattered parts of the actual world, not entire worlds at all. It would just happen that physical reality was fragmented in this remarkable but modally inconsequential way.
Gist of Idea
If other worlds exist, then they are scattered parts of the actual world
Source
Michael Jubien (Analyzing Modality [2007], 1)
Book Reference
'Oxford Studies in Metaphysics vol.3', ed/tr. Zimmerman,Dean W. [OUP 2007], p.100
A Reaction
This is aimed explicitly at Lewis's modal realism, and strikes me as correct. Jubien's key point here is that they are irrelevant to modality, just as foreign countries are irrelevant to the modality of this one.