The Cosmology of the Ethereal (GEP)

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A blood can't envision the Ethereal without fogs and mists springing to mind. That's because these vapors are the medium of the Ethereal. The fogs are everywhere and hold or border upon the Inner Planes, fledgling demiplanes, and the fully evolved Prime Material Plane. In a way, the Ethereal Plane is the cauldron that holds or borders all physical reality (as opposed to the philosophical reality of the Astral and Outer Planes). However, the Ethereal Plane is more than just a container-it's a propagator. 
The Ethereal is in some ways like a murky pool, laden with dissolved elements. A ripple on its surface is sometimes enough to cause a blob of solid ether to "crystallize," possibly forming a seed for demiplane growth. It's the mists and fogs themselves that are the visual signs of the dissolved elements constantly streaming forth from the Inner Planes.
The Inner Planes embody physical reality. They aren't concepts, but rather the concrete building blocks of every- thing "real." Just like a limited number of letters in an alphabet build a limitless number of differing sentences, the elements of the Inner Planes combine in limitless ways to create countless worlds. It is the Ethereal Plane that serves to combine and mix these elemental building blocks.
One could view the mists of the Ethereal as potential matter, unresolved but full of possibility. This is because ethereal vapors are made up of tendrils each with the potential of Earth, Air, Fire, Water, and mixtures of elemental material (from the Para- and Quasielemental planes). Thus, all "matter" on the Ethereal Plane is gaslike and flush with possibility.
When the waves of potential reality collide, they form solid ether and demiplanes. Many graybeards ultimately agree that the Prime Material Plane formed in just this fashion so long ago that a body can't even count years to measure it.