A Hollowverse is a Universe, if we can even call it that, which is an empty shell. Its ontological barrier is the only existing thing, which appears to be a bubble separating it from the Interdimensional void.
Few know in fact. It is an unknown verse for many species present in many universes, perhaps it is because it is meaningless to enter into it since there is nothing in it, either that or what causes hollowverses to be created. One possible candidate is universes going under heat death.