The NX class starship Enterprise approaches 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 made it extremely difficult to leave. The clouds were a product of the spheres found in the Expanse. (ENT: "Anomaly")
The Vulcan cruiser Seleya attempted to chart the thermobaric perimeter, but it was caught in a subspace eddy and pushed into the Expanse. They were never heard from again, until the Enterprise NX-01 found them adrift inside an asteroid field. (ENT: "Impulse")
When the sphere network was destroyed in early 2154, the cloud barrier dissipated. (ENT: "Zero Hour")