The Prime Plane

The Prime Plane is where the worlds of mortals and the Dark exist.

Geography

the Prime Material plane consisted of an infinite expanse called the Dark, an endless void filled with a fluid some call Dark Matter. Floating within the great dark are massive galaxies, each containing millions of solar systems of one or more worlds. Many of these worlds are inhabited by unique species of life, both sapient and non sapient.
While most worlds are unable to physically travel through the Dark, some such as the Eladrin (Elves), the Illithid (Mind Flayers), and other races do so frequently in vessels called Dark Ships. Some trade in peace, others battle in the expansive void in what is called Dark War.
Metaphysically wrapping around the Dark like a bubble is the Outside, which forms its outer borders. This however is a simplification; if one were to physically travel infinitely in one direction at any speed, you would never reach the Outside.

Type

Dimensional plane