The Antiverse is one of many universes within the Multiverse. As one of the first universes to be discovered by the humans, it has been subject to a huge amount of studies.
The Multiverse is the collection of all universes in the third dimension.
Superficially, it appears to be very similar to our own. The justification for its name, however, is that it contains only a fraction of the matter found in our world due to the increased particle-to-antiparticle collisions during its creation.
A slightly alternate set of physical constants has allowed it to stabilize in a similar fashion to our universe. A combination of matter quantity and universe age has caused the Antiverse to begin its transition into heat death soon after human discovery, prompting additional research of our universe's future.