Xen, or the Borderworld, is a separate overlapping dimension notably discovered by Black Mesa scientists, who regularly traveled there via teleport to collect samples and study the myriad species of flora and fauna within. It is the presumed home of the Vortigaunt race, and a place where many of the alien species that migrated to Earth during the beginning of Xeno War I originated from. It is partially merged with Byaha, and completely surrounds the universe.