The Enterprise (NX-01) is approaching the thermobaric clouds surrounding the Delphic Expanse in 2153.
A thermobaric cloud is a type of energy formation that defined the perimeter of the Delphic Expanse. The clouds were several million kilometers thick, and took at least six hours to cross at sublight speeds. (ENT: "The Expanse") The thermobaric clouds permitted entry into the Expanse, but prevented ships from leaving. The clouds were almost certainly created by the spheres found in the Expanse. (ENT: "Anomaly")
When the sphere network was destroyed in early 2154, the cloud barrier dissipated. (ENT: "Zero Hour")