An important setting and significant location in story of the Half-Life, Xen is a dimensional border world, either so unpredictable or so dangerous in its existence that even the Combine Empire will not or can not travel to it. According to Valve staffer and Half-Life lead writer Marc Laidlaw, Xen is a barren universe, with no native life originating from it. Every life form in Xen, including the Vortigaunts, Headcrabs, Antlions and even the Nihilanth itself, arrived there through artificial means, either accidentally or intentionally. It is inferred that the Nihilanth's homeworld was conquered by the Combine, forcing it to flee to the relative safety of Xen. Similarly, the Vortigaunts and Antlions also originally arrived on Xen from elsewhere, more than likely also fleeing the Combine.
In the original Half-Life, the events in the game are started by the resonance cascade caused by a crystal from Xen being put into an anti-mass spectrometer. This event opened a portal between the two worlds allowing the creatures from Xen to come to Earth. The final levels of the game actually take Gordon Freeman to Xen where he is forced to destroy the Nihilanth in order to save himself and Earth.
Xen is made up of many small floating landmasses connected by scattered teleportation portals. Xen has a much lower gravitational constant than Earth, allowing a normal human to jump tens of meters with each leap.
Study of Xen
Throughout Half-Life and the expansions that followed, the player continuously goes back to Xen, with each visit revealing more and more human machines and even dead human bodies. This suggests that the scientists at Black Mesa knew a great deal about this world before the events in Half-Life.