You may be looking for the event Collapse of the universe.

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 from the Milky Way Galaxy. As of the late 24th century, it stood as the furthest location in the universe reached by the Federation.

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 glowing cubes, sped past the ship in seemingly random patterns, in a blue nebulous background. The area would cause thoughts to become reality, as experienced by the crew of the Enterprise-D: whatever one thought actually became reality for them, a situation quickly recognized to be dangerous for anyone lacking sufficient intellectual control.

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. (TNG: "Where No One Has Gone Before")