For this series, the astral plane or in-between is where all the workings of Earth's energy occurs unseen by those living in the physical plane. As micro-organisms exist invisible to the naked eye, so do all the components of the world's energy flow. In modern day it's invisible to most mortal eyes, though there are certain individuals who are able to see and even manipulate it, an ability that Lucifer's first wife possessed as well as his secretary Gwendolyn. For example, as you talk to someone, the energy flows from you and towards the intended person. The gifted individuals or 'Espers' can see this interaction and more!
This image is to represent the incredible force that advanced espers can wield, as well as how they themselves are connected to the flow of energy.