Airibron

Airibron is a Plane of Reality located inside of the Plane Isilron. It is characterized as a universe (much like our universe) filled with galaxies, nebulas, stars, black holes, etc.

MTX-1313 (Caessian Grey)
The planets and species within Caessian Grey are largely space-faring, advanced societies that have not travleled much outside of their galaxy. Most likely the most populous galaxy of all of Airibron.