Portal might have us completing puzzles through the Aperture Science facility until we end up face-to-face with a soon-to-be potato AI, but it's also in the Half-Life universe, expanding on Valve's previous games. That was nearly much clearer, as dataminers have uncovered a potential cut ending that featured Xen's skybox.
Xen is the first Half-Life's infamous endgame dimension, the borderworld that Black Mesa uses to teleport. The general idea is that they use one portal to zap up to Xen, then slingshot back down to another point, teleporting across Earth by using Xen - this is the upper hand they have over the Combine who can't travel like that locally. But we haven't revisited Xen since the '90s unless you count Black Mesa (the fan made remake).
Portal nearly took us back there, but the ramifications of that ending would've radically changed the story of Aperture Science. As pointed out in Richter Overtime's video, there's an unused skybox called sky_escape_01_ which is based on Xen from Half-Life. It would've been used in the finale once we emerged from a cave in the facility, finding the outside to be the alien borderworld, not Earth.
Emerging in an alien dimension full of refugees on the run from the Combine would've changed Aperture's projection entirely. Right now, it's a rival corporation to Black Mesa that raced for US Government funding over its teleportation technology.
Black Mesa inadvertently invited an interdimensional alien empire to Earth's front door with its own meddling, though that's speculated to be G-Man's fault, while Aperture created and hid the Borealis while messing around with moon rocks and portal tech.
Because Aperture found a way to teleport through portals, they had no need for slingshot tech or Xen - although they might've accidentally stumbled on it like Black Mesa did. But Xen ended up being integral to Black Mesa's own tech, although this ending might've made Aperture a little closer to its rival. Still, Portal is shrouded in mystery - we don't know when it's set or where. It might be years before the Combine take over, centuries after, or even during. Xen would've only made that mystery even more potent.
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