The Delphic Expanse was an area of space approximately 50 light years from Earth. 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. (ENT: "The 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. (ENT: "Chosen Realm", "Azati Prime")

In 2153, the 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. (ENT: "Anomaly") 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 region was restored to normal in 2154 after Enterprise destroyed Sphere 41, one of the principal spheres. The destruction of this sphere caused a chain reaction, destroying the others and removing the source of the spatial anomalies that had plagued travelers within the Expanse. (ENT: "Zero Hour")

There is no known equivalent of the 'Delphic Expanse' in real astronomy - and certainly no such phenomena exists in the 50 light year distance from Earth that this Expanse is supposed to occupy. However, instead of an expanse, scientists have discovered one of the smallest planets known outside our solar system 50 light years away, orbitting the star mu Arae - a star very much like our own sun. The planet is about 14 times the mass of our earth and could be a rocky planet with a thin atmosphere. Theorists say 14 Earth-masses is roughly the upper limit for a planet to possibly remain rocky.