Dimensional Nexus

"It was like being inside joy. As if joy were something tangible and you could wrap yourself up in it like a blanket."

– Guinan, 2371 (Star Trek Generations)

The Nexus was an extra-dimensional realm in which one's thoughts and desires shape reality. Inside the Nexus, time and space had no meaning, allowing one to visit any time and any place that one could imagine. The doorway to the Nexus was a violent, destructive temporal energy ribbon which crossed through the galaxy every 39.1 years, among others in 2293, 2332, and 2371.

The above is from "Canon" on Memory Alpha. The following is for Memory Gamma, based on adventures roleplayed using Last Unicorn Games' Star Trek: The Next Generation:

The Nexus is a dimensional crossroads, that exists outside of time and space. Once there, a person can choose to remain forever, or if he chooses to leave, he can emerge anywhere, anytime he chooses. Picard did this, to return to Veridian III, just before Soren destroyed that system's sun. With the aid of James T. Kirk, Picard stopped Soren's insidious plot, and saved countless millions of lives, including the crew of the Enterprise-D.

The Dimensional Nexus was created by El-Aurian scientists Tanan Zager and Dr. Tolien Soren, from Aurellia, an M-Class planet in the Delta Quadrant of the Milky Way Galaxy. When the Borg were approaching Aurellia to consume it and assimilate its entire population, Tanan and Soren rushed their experiments into producing a permanent, if mobile, gateway to the Nexus. Once activated, the gateway drew over 1,200 El-Aurians into the Nexus, protecting them from the Borg invaders. Among these survivors were Guinan (later of the Enterprise-D), Dr. Tolien Soren, Tanan, Tanan's young son Jaryd, Tanan's brother Boothby, and many others. After creation, the gateway to the Nexus appeared as a semi-real energy ribbon, that floated through space at warp speeds, making it nearly impossible to access--that was done deliberately, so that the Borg could never reach the El-Aurians safely inside of it.

Sometime around the year 2140, the entity named Q (John Delancey's character) stumbled upon the Dimensional Nexus and the El-Aurians residing there. Q showed Tanan what would happen if the Borg ever discovered Earth, and offered Tanan a chance to prevent such a disaster. When Tanan hesitated, Q expelled all of the El-Aurians from the Nexus to Earth, in the year 2140. Most El-Aurian survivors became obsessed with getting back to the Nexus, but when that proved impossible, Tanan became instead obsessed with protecting Earth and the surrounding star systems from a Borg invasion, even though that wouldn't occur for at least several centuries. Tanan was instrumental in urging a strong Starfleet, and working with Admiral Jonathan Archer, helped draft the Articles of Confederation; indeed, it was Tanan who wrote "Section 31," which became the basis for that agency.

As Tanan was far too consumed with protecting Earth from the Borg, his 12-year-old son Jaryd was left to be raised by the boy's uncle Boothby, and his godmother, Guinan. Jaryd eventually chose the surname Harker, which was appropriate for a member of a race of "Listeners," and began a medical practice in San Francisco. Jaryd later joined Starfleet Academy on the advice of a joined Trill officer named Kahn, and the organization benefited greatly from his two centuries of medical experience. Harker served for years as Chief Medical Officer aboard the USS El Dorado commanded by Captain Jose Sanchez, and became a medical agent for Starfleet Intelligence, under Admiral Nyota Uhura. Jaryd Harker later was promoted by Admiral Uhura to full commander, then to captain, and given command of the USS Twilight NCC - 74413, a medical research vessel.

Guinan, after years of drifting from profession to profession, eventually ended up tending bar at Ten Forward on the USS Enterprise 1701-D. There she took part in a number of adventures, and was a confident and advisor of Captain Jean-Luc Picard. At one point, Guinan was instrumental in helping Picard put time back on the proper track, when the Enterprise-C passed through a temporal rift in space, an event that changed history. Guinan insisted that the ship from the past go back, and when it did, the Enterprise C returned to the battle of Nervalla III, defending a Klingon Colony there; the ship was lost in battle with the Romulans, and her crew taken prisoner, but the act of bravery and honor could not be ignored by the Klingon High Council. Because of the Enterprise-C's bravery and sacrifice, relations between the two powers were cemented, and the Federation and Klingon Empire are fast friends and allies.

Boothby became a quiet, unassuming gardener, tending the grounds of Starfleet Academy, and molding the lives of officers who would one day captain Starfleet's vessels. Among these fine young officers were Captains Picard and Janeway, and Commander Chakotay.

Tanan became the Director of Section 31, once a branch of, and later a rival agency to Starfleet Intelligence. Under Tanan's leadership, Section 31 became a ruthless organization that protect Earth and all member worlds of the Federation, whatever the cost. Tanan feels that his son Jaryd has some growing up to do, before he joins Section 31 and helps protect the UFP in ways that Starfleet wouldn't possibly consider. For his part, Jaryd is fiercely loyal to Uhura, and embarrassed and ashamed that his father could conceive of mass murder and genocide, even to protect the Federation.

Since there are so few survivors of Aurellia and the Nexus, all El-Aurians know each other. While the El-Aurians naturally have a lengthy lifespan of several thousand years, it is uncertain whether their sense of Time being altered, and sensing the presence of Q, and other beings who can alter time, are theirs naturally, or a result of spending so much time in the Nexus.