Some notes on Cosmology

At the end of the last segment of the campaign, the party obtained access to a vessel that can travel in the Astral Sea.  Given that fact, I thought I should set down some of the facts about the cosmology of my game universe.  The D&D cosmology has varied quite a bit from edition to edition.  I've never really fully embraced any interpretation.  I haven't done much gaming that left The Material Plane, so  I've never had to work all this out.  Here are my thoughts as of now.  I reserve the right to make changes.

The center of the universe is Limbo.  Limbo is a realm where matter and energy behave in a random manner.  Only creatures with powerful magic can carve out stable realms within Limbo to survive there. 

As Limbo expands outward, it becomes gradually more orderly.  Eventually it reaches a boundary where matter is stable.  This is the Astral Sea.  The Astral Sea is much like "space" in the real universe, except that fairly small objects will spontaneously acquire gravity and a breathable atmosphere.  If you find a raft floating in the Astral Sea, you can stand on it.  (Actually you can stand on both sides.)  You can breath and you won't freeze to death or explode from lack of pressure.  You won't be killed by waves of radiation. 

There is no Material Plane, per se.  Within the Astral Plane there are floating spheres of stable matter.  These spheres tend to exist in groups, much like the stars and planets in our reality.

Each of these Material Spheres (call them planets if you like), exists in 3 overlapping layer.  There is a Natural World. Along side it there is The Feywild.  This is a more magical reflection of the Natural World, sometimes called the Realm of Faeries.  The 3rd material layer is called Shadowfell.  It is a dark and dismal shadow of the Natural World.  These 3 layers exist simultaneously and occupy the same position within the Astral Sea. 

Ordinarily, the existence of the other layers is completely undetectable.  But there are places where the boundary becomes blurred.  In such places, you may cross from one layer to another without even realizing it.

Beneath the surface of the Natural World, breaches between layers are much more common than above ground.  Huge caverns and complex mazes exist in a realm that mixes magic and shadow.   This area is know as The Underdark.

Beyond the edge of the Astral Sea lie many more planes of existence.  These are places where the laws of nature do not apply.  These include the Elemental Planes and all conceivable "heavens" and "hells". 

Generally speaking, the effects of magic are confined within a single plane or material sphere.  You can not use an ordinary teleport spell to travel from one planet to another, for example.  There are, of course, powerful spells that can give direct access to many planes without a protracted journey across the Astral Sea. 

With a vessel that can sail upon the Astral Sea, you can in theory travel to any point in the cosmos.