The Cassiopaea material discusses the concept of an antimatter universe in various places.
This universe is said to interact with the matter universe through gravity. It is implied that the antimatter universe is not simply a mirror image of the matter universe in the sense of CPT symmetry but something quite different. The antimatter universe as used in the Cassiopaea material has to do with passage between densities, space-time manipulation and a world of consciousness/energy as opposed to a world of matter. It is difficult to place these references in the context of what is known of antimatter In physics.