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Currently in HL2 three dimensions are known to exist.
-The Combine Home Dimension
-Xen
-Earth
Earth exists as one dimension. The resonance cascade allows travel between Xen and Earth. The Xen reach our dimension via the Nihilanth, and we reach Xen via our machinery. Before this, the Combine had also attacked Xen. The Xen races created the Nihlanth to stand strong against the Combine, and ended up being enslaved by the Nihilanth. We destroy the Nihilanth. Xen goes into chaos and the vorts go to Earth to flee. The combine pass through Xen and then takeover Earth. You arrive on the scene, raise all sorts of hell, and the game ends there.