Board Thread:Questions and Answers/@comment-25524514-20141012233028/@comment-5553481-20141014154357

Imagine taking dreams of EVERYone, throwing them into a mixer, and TA-DA, the Astral Realm/Plane.

In short, the Astral Plane is the "collection" of everyone's imagination, and can be altered easily.

Note: other people who Astral Project, ARE in fact real, the only problem is finding out if they're actually people and not humanoid beings.