An energy ribbon that crosses the galaxy and leads into a timeless realm where any wish seems real, central to Star Trek: Generations.
The Nexus is a ribbon of energy that travels on a long loop through the galaxy, passing through the same region of space roughly every thirty-nine years. Anyone drawn inside enters a timeless realm where they experience whatever they most desire, and those who have been there long to return.
In 2293, the ribbon destroyed two El-Aurian refugee ships, and the Enterprise-B rescued some of the survivors, including Guinan and Tolian Soran, as they were pulled out of the Nexus. Kirk disappeared when the ribbon struck the ship and was taken into the Nexus.
Soran spent the next decades trying to get back, and in 2371 he destroyed stars to change the ribbon's path so that it would pass by Veridian III. Picard was swept into the Nexus there and met an echo of Guinan, who told him how to leave. He persuaded Kirk to come with him, and the two returned to the moment before Soran's launch to stop him.