The Negative Zone is another dimension that was discovered by the Fantastic Four. It’s another universe, parallel to ours, that is nearing it’s natural heat-death, and there isn’t much life left anywhere. The few who do still survive are forced to live in a harsh universe, living brutal lives. Also (and I could be wrong here) not all of the constants and rules that apply to our universe apply to theirs. For example, there is a breathable atmosphere throughout the whole thing instead of empty space.
When Spider-Man traveled there in one adventure, his costume changed colours and became this black and white variation. Also while he was there, he met and teamed up with someone who eventually gave him a new costume that didn’t absorb any light at all, and let him blend in with shadows. This would eventually become the Dusk costume later on.
Positively Jim 