Null space is an abnormal region of space. The pocket of space is filled with the bright light of condensed turbulent magnetic and gravitational fields absorbing all electromagnetic energy from anything that enters the phenomenon. The fields also bend all outside energy around the pocket, making it naturally cloaked.
According to a hypothesis formalized by J'naii specialists and Commander William T. Riker, these pockets may be naturally created during the formation of star systems, when clouds of interstellar dust and gas coalesce, giving rise to turbulent regions that under certain circumstances condense into these null pockets. Geordi La Forge further theorized that the size of the pocket may be linked to the rate of energy absorption within and that the rate of absorption might not be constant within the anomaly.
As of 2368, the existence of null space was only theoretical, until encountered by the USS Enterprise while attempting to help the J'naii recover one of their shuttles, Taris Murn, which had become lost somewhere inside their star system. Realizing that the shuttle had entered an irregularly-shaped null pocket of space, Riker collaborated with a J'naii shuttlecraft pilot named Soren to map the null space. Using a series of photon pulses, they were successful in charting the pocket's perimeter, and piloted the Starfleet shuttle Magellan inside.
While inside, Riker and Soren located the lost J'naii shuttle, but found it difficult to beam the two J'naii survivors aboard due to the energy absorption in the pocket affecting the annular confinement beam of the transporter. They were able to beam the survivors aboard with a second try, but found that they were not able to beam themselves back to the Enterprise. In a last-ditch effort, Riker channeled all on-board energy into the transporter and energized. Although this action had the effect of destroying the Magellan, Riker, Soren, and the two J'naii survivors were able to successfully beam back to the Enterprise. (TNG: "The Outcast")
Null space could also be used as a medium for transportation. In 2376, the starship USS Voyager encountered a space catapult in the Delta Quadrant, constructed by the crew of Tash's starship. Having been stranded at least ten years away from their homeworld while exploring an unstable wormhole, they built the catapult in an attempt to return home. It operated by locking onto a vessel with a graviton surge that propelled it to submerge into null space. The vessel would then emerge thousands of light years away after only hours of travel. When null space was used this way, an unprepared ship was found to be more likely to encounter system overloads, not energy absorption. After Tash was successful in using the catapult to travel five thousand light years, it was utilized by the Voyager crew to eliminate three years off of their journey back to the Alpha Quadrant. (VOY: "The Voyager Conspiracy")