An Antiverse is practically a universe full of anti-matter instead of your usual normal matter. Physics here might be different or still the same, Beings from this universe will think that the anti-matter they have is normal matter and our universe is anti-matter. It's size is 1,2 septillion light-years. Antiverse applies to all verses below the Voidverse i.e. Antixenoverse, Anti-etc... The largest antiverses are the AntiUNNAMEDverse (aka the unkown antiomniverse).
The antiverses are outside of the UNNAMEDverse but, they contained by the Absoluteverse.