Wildspace

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

Geography
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.

Any object or creature leaving a planet's atmosphere carried with it a naturally-occurring air envelope, whose size depended on the size of the object: usually an air envelope extended out to a radius equal to the object's diameter. The air carried in this fashion could be used by creatures inside to breathe. In case of a typical human, the size of their envelope allowed for at most a few minutes of breathable air. The envelope around a large ship, however, could support a number of creatures for several weeks.

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.