The site of the Xindi hunt.
The Delphic Expanse is a dangerous region of space where unusual physical phenomena, known as spatial anomalies, are common. Ships that enter it find their sensors disrupted, their warp drives strained and their crews exposed to a variety of strange hazards. It was a hostile place for explorers.
The expanse was created by the Sphere Builders, powerful beings from another dimension, who used the spheres within it to alter local space and make it suitable for them. Their goals required the transformation of the region, and the anomalies were a side effect of that effort. A number of species lived within the expanse, including the Xindi.
The Enterprise was sent into the Delphic Expanse in the third season of Enterprise, after a probe launched by the Xindi attacked Earth. Captain Archer and his crew spent a year there, trying to find the Xindi and prevent a second attack. The mission tested the ship and its crew, and led to a number of memorable episodes, including the discovery of the deceit of the Sphere Builders.