[volantis]

There are already two known physical Universes, that of normal matter and that of antimatter. Due to the half spin nature of subatomic particles, both of these types of Universes occur in the forward direction of time. According to the Aether Physics Model (described on this site's research section) there are also matter and antimatter existing in the backward time direction. Thus there are four and exactly four different Universes.

Since the photon is exactly the same for matter and antimatter, it could very well be that many of the galaxies we see in deep space are actually made from antimatter.

One could make the argument that since we can see antimatter, then it must exist in our own Universe, and therefore there is only one Universe. But we have to draw a line somewhere as to what constitutes a boundary between our Universe and another Universe. I draw this boundary according to the structure of the subatomic particles, which make up the matter in each Universe. Subatomic particles with left spin, forward time direction (our "normal" matter) constitutes a Universe. Subatomic particles with right hand spin, forward time direction (antimatter) constitutes a different Universe. The two Universes can share the same space-time fabric (Aether), but the two different types of matter cannot coexist together. According to the APM, matter and antimatter are gravitationally repulsive, so an antimatter galaxy will usually steer clear of a normal matter galaxy.

The first question we must answer before asking if there are multiple Universes, is what defines a Universe?