 Such dimensions, such as Asgard or the Dark Dimension are not "pocket dimensions" as they reside completely "outside" the boundaries of the Marvel Universe instead of within, as the former does.One cannot normally alter the Marvel Universe's history; if a time-traveler should cause an alteration to the established flow of events at some point in the past, a divergent universe will simply "branch out" from the existing timeline, and the time-traveler will still return to his or her unaltered original universe.Those realities can also spawn realities of their own.