White hole

A white hole was an astronomical phenomenon. It was a ruptured singularity, the opposite end of a black hole.

A particle fountain white hole emitted matter and energy originating from another dimension or universe. A vortex white hole was a doorway to another universe.

Background
A particle fountain white hole looked like a spew of geysers of white gold, resembling a spindly chrysanthemum. The matter erupting from a white hole was subject to intense gravitational and temperature variations, which fundamentally altered its atomic structure. As the matter cooled in vacuum, it condensed into asteroids and dispersed into a spherical debris field. Gravitational forces ebbed and flowed, producing navigational hazards. 

History
In 2274, the USS Enterprise (NCC-1701) pursued the USS Icarus (23rd century) through a vortex white hole in the Ilyria system into an exotic dimension. Captain Kadan and his Phaetonian crew had been driven mad by the Galactic barrier. Spock detected life signs near some very unusual-looking matter. James T. Kirk fired phasers at Icarus to divert the crew’s attention from the local life forms. Both ships re-emerged into normal space 200 parsecs away from the white hole. 

In 2366, Starbase 173 detected gravitational tidal forces that led the USS Enterprise (NCC-1701-D) to discover a white hole a few million kilometers away from the Romulan Neutral Zone. Matter emissions were positively charged, leading to a speculation that the universe on the opposite side had similar laws of physics. Energy emissions dramatically increased, and then the white hole disappeared. The energy burst deactivated shields, deflectors, sensors, and warp and impulse engines. Gravitational tides dragged the helpless starship into the Neutral Zone. 

Three non-corporeal sentient beings had been drawn through the white hole and into the Enterprise. In order to learn about the Enterprise’s universe, they probed the minds of the crew, but inadvertently impaired the crew’s judgment. To make amends, they visited the crews of two Romulan warbirds in the Neutral Zone, including Tomalak's warbird. 

In the 2370s, the Federation began to experiment with mining white hole matter at Tyrus VIIa.

In 2376, with the help of Chieftain J'Maah and Science Minister M'Yeoh, Elias Vaughn traded with the Consortium to obtain exotic material mined from a white hole. The Consortium used small ramscoops to gather uncooled particulate matter before it hardened. Larger asteroids within the debris field of the white hole were used for habitation, resulting in the establishment of a complex, networked city-state. 

Known white holes

 * Consortium white hole
 * Ilyria system white hole
 * Romulan Neutral Zone white hole
 * Tyrus VIIa