The Negative Zone is a universe with laws of physics that are very close to our own. There are stars, planets, galaxies, and life much along the same lines as ours. There is one major difference: everything in the Negative Zone is composed of antimatter; that is, matter composed of subatomic particles that have the opposite electrical charge (positrons instead of electrons, anti-quarks instead of quarks, etc). Since matter and antimatter react explosively with each other - the explosive force of a few grams of antimatter reacting with regular matter is often said to be equal to a few tons of TNT - it is incredibly difficult for any person or object to enter the Negative Zone without precautions.
The existence of an antimatter universe has been theorized for decades. It wasn't until an explosion nearly destroyed the Large Hadron Collider during a routine experiment that direct physical evidence was obtained. As one of several dozen scientists asked to examine the data obtained, Dr. Reed Richards (who was given the data while working for NASA as he worked on his Fantastic Voyage project) of the Fantastic Four came to the conclusion that the explosion required a greater amount of antimatter than could be produced by the particle accelerator, and that the experiment must have opened a temporary aperture to an antimatter universe. Inspired by this conclusion, Richards wondered how one might explore such an almost certainly fatal universe. It was an off-hand comment by the least educated member of the Four, Johnny Storm, that shut Reed in his laboratory for three days to develop a means of turning matter into antimatter - and vice verse - during the dimensional transition without the expectant destruction.
At first the Fantastic Four sent unmanned probes through a stabilized portal Richards set up in the Baxter Building, which when returned to Earth revealed a universe not unlike their own. Inspired to do some first-hand exploration, Richards installed a Negative Zone projector on their starship, the Fantastic, and the Four for the first time visited the Negative Zone.
It was on this first visit that they encountered Annihilus, a tyrannical alien despot native to the Zone who sought to conquer or destroy all life in the Zone. A subsequent visit also netted them them the emnity of Annihilus's rival Blastaar. Both Annihilus and Blastaar have sought to conquer Earth, seeing it as a useful source of slave labor. As a result, Richards has sealed the now-permanent portal in the Baxter Building behind a door of solid adamantium (see Common Technologies).
Officially Richards and the Fantastic Four have handed off further exploration of the Negative Zone to NASA and other space agencies, who have mostly decided on a very conservative approach, producing only a handful of unmanned scientific probes to explore it. The Four remain the acknowledged experts on the Zone, however, and receive copies of all data gathered. They also occasionally venture into the Negative Zone during the course of investigations or when asked to provide more information on the hostile forces there. At the same time, various manufacturers have quietly obtained examples of Annihilus's technology and are working to reverse-engineer it.
According to Richards's research, multiple antimatter universes are possible; the one designated the Negative Zone is the only one charted so far.
First Post-Reboot Appearance: FANTASTIC FOUR #