The realm outside normal space from which the Sphere Builders come, whose physical laws the great spheres of the Delphic Expanse are slowly imposing on the galaxy.
The transdimensional realm is the home of the Sphere Builders, the hidden instigators of the Xindi attack on Earth. They cannot survive in ordinary space, so they seeded the Delphic Expanse with a network of enormous spheres that reshape the region into something they can inhabit.
In Harbinger, Enterprise recovers a dying alien from a pod inside a field of anomalies; he is a test subject sent to see whether his kind can yet live in the altered space. From their own domain the Builders look across possible futures, as seen in The Council and Countdown, and steer the Xindi through the figures the Reptilians call the Guardians.
In Zero Hour, Enterprise destroys the controlling sphere. The network collapses, the Expanse returns to normal and the Builders who have crossed over perish.