Thirdspace

Thirdspace was a "third" dimensional plane (normal space being first and hyperspace being second) which would theoretically reduce travel times by the same factor as hyperspace compares to normal space travel. (This theory was never proven.) The dimension was first accessed by the Vorlons, who had convinced themselves they were gods and sought to ascend to a divine realm. Instead, upon opening the portal they had created with a titanic device, they were attacked by a race of malevolent, highly psychic aliens bent on destroying all life besides their species. The aliens reworked the gate to magnify their mental powers, and were even able to turn some susceptible Vorlons to their side. The Vorlons were able to seal the gate at great cost, but the aliens' worshippers were able to hide it within hyperspace.

The Vorlons introduced a racial memory warning their descendants about Thirdspace and the gate. However, the gate was rediscovered shortly after Babylon 5 declared its independence from Earth, and Interplanetary Expeditions quickly sent a team to investigate. However, the aliens' influence began to affect some of the B5 and IPX denizens, and eventually the IPX chief, Dr. Trent, was able to reactivate the gate. A pitched battle between B5 forces and Thirdspace aliens (and their slaves) ensued. John Sheridan was able to set off a nuclear bomb within the workings of the gate, destroying it and sealing off the portal to Thirdspace permanently.