A center halo was a stellar mist where all dimensions met and time ceased to be. They were theorized to exist by an Ancient named Delik who believed that they were scattered throughout the universe. Initially within the halo, at least within certain regions, time appears to only pass at a slower rate than in the rest of the universe. Eventually though, the halo hardens and everything inside is trapped, without time, forever.
Moya and her crew encountered a center halo and, not knowing of its nature, attempted to hide inside it as protection from the Peacekeepers. Once they'd entered, navigation was highly difficult with John Crichton remarking that they could not see more than 300 metras. ("The Locket")