Svartalfheim is known as the seventh world of Asgard. It exists on an extradimensional plane that consists of major land masses which can be reached via nexus portals that exist on each of these worlds except for Midgard, and some of these worlds are in direct contact with the roots of Yggdrasil, the cosmic ash tree that stands in Asgard.
Thor #347 (1984)
Svartalfheim is the seventh of the nine worlds of Asgard and home of the Dark Elves, who are currently ruled under the peaceful edict of queen Alflyse. However, before Alflyse, Svartalfheim was governed by the power driven Malekith whose sole purpose in life was the conquest of all nine worlds of Asgard.