The destruction in 2154 of the network of giant artificial spheres that created the Delphic Expanse, achieved by Enterprise NX-01 disabling a key sphere and triggering a chain reaction.
The spheres were moon-sized constructs placed by transdimensional beings known as the Sphere-Builders, or to the Xindi as the Guardians. Their gravitational fields generated the spatial anomalies of the Delphic Expanse and were slowly remaking the region so that their makers could live in it.
In Zero Hour, the finale of the third season of Enterprise, T'Pol takes the ship to Sphere 41, a control node for the whole network, while Archer pursues the Xindi weapon. The crew endure the transformed space around it with Phlox's help and fire a modified deflector pulse into the sphere.
It collapses, and the others follow. The anomalies vanish, the Sphere-Builders are driven out, and the Expanse returns to normal space.