Vast spheres built by the Sphere-Builders, whose network reshaped the Delphic Expanse to suit transdimensional life until Enterprise destroyed Sphere 41 in Zero Hour.
The Delphic Expanse spheres were enormous artificial structures, each many kilometres across, scattered through the Delphic Expanse. They were built by the Sphere-Builders, beings from another dimension, and together they formed a network that was slowly altering the laws of physics in the region, creating the spatial anomalies that plagued travellers there.
The Sphere-Builders hoped to make the Expanse habitable for themselves and then spread further. Having seen a future in which humanity defeated them, they manipulated the Xindi into building a weapon to destroy Earth. In The Council, T'Pol led a team inside a sphere to retrieve data from its core.
In Zero Hour, while Archer fought to stop the Xindi weapon, T'Pol took Enterprise to Sphere 41, a critical point in the network. Despite a distortion field and attacks by the Sphere-Builders, the crew destroyed it with a modified deflector beam, setting off a chain reaction that destroyed every sphere and ended the anomalies of the Expanse.