The Delphic Expanse was a region of space approximately fifty light years away from Earth. (ENT: "The Expanse") According to the Triannons, a native species which regarded the Expanse as the "Chosen Realm," an area that had been created by supernatural beings they referred to as "the Makers," in preparation for their eventual return. (ENT: "Chosen Realm")
Nearly two thousand light years across, the Expanse was riddled with a web of dangerous spatial anomalies and was surrounded by a dense layer of thermobaric clouds, which made navigation extremely hazardous for starships. In some regions of the Expanse, the laws of physics did not function reliably. Artificially created by transdimensional beings using a vast number of massive spheres, the Delphic Expanse ceased to exist in 2154, when the Earth starship Enterprise NX-01 destroyed them. (ENT: "The Expanse", "The Xindi", "Zero Hour")
In 2133, a Klingon vessel attempted to enter the Delphic Expanse. When it reemerged, the entire crew was anatomically inverted but still alive. After this incident the Klingons would not enter the Expanse. Vulcan ships had also entered the Expanse, but only a few had ever returned. (ENT: "The Expanse")
In December of 2152, the Vulcan ship Seleya embarked on a mission to chart the thermobaric clouds surrounding the Expanse. After several days the ship's crew reported that the Seleya was caught in a subspace eddy and being pulled into the Expanse and contact was lost. The crew of the Vulcan ship Vaankara, sent to search for the lost Seleya, went insane, killing each other and self-destructing the ship after only two days inside the Expanse. After this the Vulcans considered any expedition into the Expanse to be a foolishly risky action. The Enterprise later found the Seleya adrift, with its crew gone insane due to trellium-D exposure. (ENT: "The Expanse", "Impulse")
When Earth suffered a devastating attack which resulted in the deaths of seven million people in March of 2153, Captain Jonathan Archer received intelligence from the Cabal's mysterious benefactor that the attack had been launched by the Xindi, a race native to the Expanse. With this information, the Enterprise embarked on a mission into the Expanse to find the Xindi, in an attempt to prevent another attack which would destroy Earth. (ENT: "The Expanse")
While in the Expanse, Enterprise encountered the Triannon and discovered that their myth was based in fact, and that the Expanse had been created by a race of transdimensional beings who were attempting to reconfigure space to make it habitable for their species as a prelude to invasion. (ENT: "Chosen Realm", "Harbinger")
The crew discovered that this race had constructed a series of massive spheres, each one of which emitted powerful waves of gravimetric energy, which were responsible for the spatial anomalies. Possessing advanced temporal technology which allowed them to examine alternate timelines, the Sphere-Builders discovered that, in the 26th century, the Federation was destined to finally defeat the Sphere-Builders and drive them back into their transdimensional realm. In an attempt to prevent this future, the Builders contacted the Xindi and led them to believe that Humanity would eventually be responsible for the destruction of their homeworld, and that the only way to save themselves was to destroy Earth first. (ENT: "Azati Prime")
In early 2154, Captain Archer was eventually able to convince the Xindi that they had been manipulated, and entered into an alliance against the Sphere-Builders. Enterprise was successful in disabling one of the Spheres with a deflector pulse, causing a chain reaction which destroyed the entire Sphere network. This effectively caused the Expanse to cease to exist, and the thermobaric cloud perimeter dissipated. (ENT: "Zero Hour")
- Tarquin's species
- Tret's species
The size, mentioned by Soval, presumably did not refer to a spherical diameter of the expanse, as such a huge phenomenon would be a dominant feature within the galaxy. If the size is accurate, it would suggest the expanse was a somewhat flat region of space.
In the final draft script of "The Expanse", the Delphic Expanse was said to be nearly a light year across, rather than almost two light years.
According to Stellar Cartography: The Starfleet Reference Library ("Stellar Cartography" p. 17; "Federation Historical Highlights, 2161-2385"), the Delphic Expanse was located in the Beta Quadrant. This expanse was "angled some 2,000 light years from "top to bottom" within the 10,000 light year thickness of the galactic disk".
A form of the Delphic Expanse also appears in the alternate reality in Agents of Yesterday, the third expansion to Star Trek Online. In the mission "Terminal Expanse", the player character from the "main" timeline – tasked by Agent Daniels to battle the Sphere-Builders and their mysterious 'envoy' – arrives in the alternate reality in the year 2260, and joins with Science Officer 0718 and the USS Yorktown to stop the Sphere-Builders from creating an Expanse in that universe.
"Terminal Expanse" also indicates that the Sphere-Builders have attempted to create Expanses in numerous realities, all with often catastrophic results.