Wildspace

Wildspace was the empty vacuum between astronomical bodies inside a crystal sphere.

Description
The boundaries between a planet's air envelope and wildspace were gradual. As altitude increased, air became increasingly thinner until it was considered vacuum. Most crystal spheres did not have an internal atmosphere and were filled with wildspace instead.

Crystal spheres appeared to have the property of maintaining the emptiness of wildspace inside them, preventing the phlogiston from penetrating through any portal that was opened on their surfaces.

History
The wildspace of Realmspace was unusually warm when compared to other crystal spheres. It was believed that the reason for that was that crystal spheres grew warmer as they aged, in which case Realmspace was probably the oldest of the known crystal spheres.