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.
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.