The Negative Zone
No one in our universe knows how or when the Negative Zone appeared or how old it is. If its aging process is similar to our universe’s, it is much older: it has already begun to contract and will eventually implode (a Big Crunch as opposed to a Big Bang). At one time there appear to have been several species who had flourishing cultures and made progress in science and the arts. When the contraction began (one estimate puts it at a million years ago), these species seem t have disappeared.
The Negative Zone is an alien universe with several characteristics that distinguish it from our own. First, and most important, it is composed entirely of anti-matter. Anyone or anything that moves from one universe to the other must reverse its polarity on a molecular level or be instantly annihilated. Second, it is a mature universe that has already started to contract (ours is still expanding); eventually, it will implode. Third, time passes at a faster rate relative to ours. Fourth, a still unknown factor (possibly lack of water) makes evolution difficult, because much of the Negative Zone is uninhabited.