Underspace

The Underspace, also known as the Vaadwaur Subspace Corridors(Star Trek: Star Charts), was a network of subspace corridors in the Delta Quadrant that allowed starships to travel great distances in a matter of minutes. 

History
Prior to the 15th century, the Vaadwaur spent 900 years mapping the corridors of the underspace. The underspace took them as far as Talax and beyond. They used them at first for trading and exploration, they also used them to expand their empire by attacking undefended worlds at their pleasure. The Vaadwuaur kept no written records of the Underspace, rather keeping the information committed to their memory.

The Vaadwuar, however, lost control of the underspace when the Turei and a dozen other species rose up against the Vaadwuar. After apparently destroying the Vaadwuar homeworld, The Turei took control of the underspace. Unfortunately, they did not have the Vaadwaur's knowledge as to how to maintain it.

In 2376, the Underspace was discovered by the Federation starship Voyager when Voyager's warp field interacted with one of the corridors, and it was pulled into the Underspace. The Turei expelled them from the corridor with a resonance pulse, but then demanded to board Voyager to eliminate all evidence of the Underspace. Voyager took shelter on the radioactive Vaadwaur homeworld, where they discovered a batallion of Vaadwaur who survived the war in stasis pods.

The Vaadwaur offered to cooperate with the Voyager crew and show them corridors of which the Turei have never been aware. However, their desire to regain their former power ultimately led them to attempt to capture Voyager and its advanced technology. Voyager defeated them with help from the Turei, and the remaining Vaadwaur ships fled into the Underspace.