The Delphic Expanse was an area of space approximately 50 light years from Earth in the Alpha Quadrant. While its exact dimensions were unknown, Vulcan scientists estimated the Expanse to have been about 2,000 light years across in the 22nd century. At that time, it was also surrounded by thermobaric clouds that made it difficult to enter and exit the region. It was generally avoided by Vulcan ships because of various spatial anomalies existing inside; in some parts of the Expanse the known laws of physics did not seem to apply.
In the 2130s, a Klingon ship emerged from the Expanse with its entire crew anatomically inverted, but still alive. After this incident the Klingon High Council forbade all ships from entering the Expanse. However, Klingon Captain Duras, during his attempt to capture Captain Jonathan Archer, risked entering the Expanse to do so. Unfortunately for him, his ship was destroyed just short of entering the Expanse.
The Vulcan ship Seleya was pulled into the Expanse in December 2152, with all contact subsequently being lost. The crew of the Vaankara, sent to investigate, went mad and killed each other within two days of entering the Delphic Expanse. The Triannons, a species native to the area, called the Expanse the "Chosen Realm". They believed that it had been created by supreme beings they called the Makers as a paradise for their eventual return. They thought the Makers were using the anomalies to reshape the Chosen Realm into a new form. Events would eventually show that the Triannons were essentially correct.
In 2153, Enterprise (NX-01) entered the Expanse in their search for the Xindi, a race native to this region, who were believed responsible for an attack on Earth that killed seven million Humans. On this mission, the crew learned about the existence of ancient spheres scattered around the Expanse, which were responsible for the spatial anomalies and ultimately the Expanse itself. The Delphic Expanse had been created by the Sphere Builders as part of the Temporal Cold War; the Expanse was designed to continue growing indefinitely, gradually altering the space-time continuum in the Milky Way Galaxy to make it habitable for their kind. They were destined to be defeated at the Battle of Procyon V by the United Federation of Planets in the 26th century. The Xindi attempt to destroy Earth was also part of their plot to manipulate the timeline, implemented to prevent the founding of the Federation, and thus their eventual victory at Procyon.
The Delphic Expanse ceased to exist when the space within the Expanse was restored to its natural state in January of 2154. The Enterprise destroyed Sphere 41, an important link in the Sphere network. The destruction of this Sphere caused a chain reaction, destroying the others and removing the source of the spatial anomalies that had plagued the Expanse for centuries.
- Tarquin's species
- Tret's species