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The end of the universe, where none have gone before, and the outer rim were informal euphemisms used by Captain Jean-Luc Picard of the USS Enterprise-D to describe an unknown area over one billion light years away from the Milky Way Galaxy. As of the late 24th century, it stood as the furthest location in the universe reached by the Federation. (TNG: "Where No One Has Gone Before") Lieutenant Commander Dexter Remmick called the location the edge of the universe. (TNG: "Coming of Age")
After leaving the Triangulum Galaxy, which was itself reached due to a mistake on the Traveler's part during a warp engine experiment, another mistake caused the vessel to not only leave Triangulum, but arrive one billion light years away from the Milky Way. In the unknown area, unidentified objects of huge size, resembling transparent cubes with bright glowing corners, sped past the ship in groups with seemingly random patterns, in a blue nebulous background.
The thoughts, imagination and memories of the crew began to manifest as subjective or shared reality. Jean-Luc Picard's mother, who came to Picard in a vision, suggested they might in fact be in the universe's beginning, rather than end. The Traveler however explained that "thought" was in fact the basis of all reality and the essence of the area where they were. A situation quickly recognized to be dangerous for anyone lacking sufficient intellectual control. (TNG: "Where No One Has Gone Before")