These are naturally occring thin tunnels of normal space that snake and shift through sub space randomly throughout galaxies. They are he most dense (and stable / large) in areas with multiple heavy gravitational fields. Some corridors are no larger than a few meters while others can be as large as well several hundred meters and tend to follow randomly shifting paths along gravitational currents connecting globular clusters, multi-star systems, and dense areas of sub space instability.
Internal Distortions
Inside a corridor space is heavily distorted and matter entering a corridor can be pushed along its length at incredible speeds exiting at a random distant point. These distortions tend to tear matter apart at a sub-atomic level. Unzipping electrons from orbits and scattering apart nucleic particles.
Interaction with Normal Space
The corridors naturally breech into normal space at random along their shifting and unpredictable course through space, but it is very rare. There are scattered instances of ships being caught in a corridor and traveling hundreds of lightyears in mere seconds. This is almost always coupled wih extreme hull damage and high levels of radiation exposure to the crew.
Theoretical Usage
A paper was published by a Farenghi scientist reporting to have proof that a space craft could detect and enter a corridor at will and use its deflector dish along with the warp drive to navigate these corridors. According to the theory and corridors could be wedged open by a carefully modulated graviton beam directed toward key points that shift and move along the walls of the corridor. The beam would have to direct powerful gravitons in short pulses at these points to keep the corridor from collapsing around the ship, crushing it.
Controversy
The paper had major flaws in the statistical analysis and the raw data used to generate it. The scientist in question then patented the idea under galactic law with every major power in the Alpha and Beta quadrants. The science is considered to be moderately sound but the process has never been proven or studied (due to legal issues). It is generally considered to be a laughable and very dangerous line of research with almost no hope of payoff.
Rumor
Rumors have it that the technology to actually traverse space using graviton corridors does exist and does actually work as reported but no evidence it has ever been found.