The Sphere Builders are a race of aliens that live in a plane of existence separate from our own dimension. They constructed a series of artificial spheres to create the Delphic Expanse, reconfiguring space to be more habitable for their own purposes.
In an alternate timeline, in the 26th century, the Lethean's, out of spite against the Phoenix-X, claimed they were the ones to call upon the Sphere Builders to attempt to inhabit our universe. (Star Trek: Phoenix-X: "Temporal Doom! Part II")