The Dead Zone is a void of darkness created by Garlic Jr. It sucks people into it and traps them floating through endless darkness. Garlic Jr. opened it twice. In the movie, Dead Zone, he opened it in an attempt to suck in Goku and Piccolo, knowing that despite his immortality he could not beat them. But Gohan was able to resist its suction and knock Garlic Jr. into the Dead Zone. He remains there for about three years until the Makyo Star gets close enough to Earth, allowing him to escape from it. During the Garlic Jr. Saga in the anime, Garlic Jr. reopened the Dead Zone in retaliation to his dark plans being foiled (Kami and Mr. Popo had broken his curse on the people of Earth using the Sacred Water). Piccolo, Krillin, Kami, and Mr. Popo were nearly sucked into the Dead Zone, but Gohan destroyed the Makyo Star, draining Garlic Jr.'s fearsome strength and causing him to be sucked into the Dead Zone once again. By the end of Dragon Ball Z (though it is never referenced following the Garlic Jr. Saga, save for maybe Piccolo's line of the Lookout's being completely ruined again) he still lives, immortal, and is still trapped inside the Dead Zone never to escape.
Both times Garlic Jr. was sucked into the Dead Zone, the void closed up.It is interesting to note that in Episode 93 (108 in the uncut), The Heavens Tremble, Garlic Jr. says he was imprisoned inside the Dead Zone for ten years, while approximately three years have gone by between Dead Zone and the Garlic Jr. Saga. However, as the narrator described the Dead Zone as "a timeless spaceless prison" in Episode 101 (116 in the uncut), Brief Chance for Victory, it is possible that the Dead Zone, like the Hyperbolic Time Chamber, takes up more time than the outside world does. It might also be possible that Makyan years are shorter than Earth years, like Namekian years are.