Wildspace is where all the planets are; each planetary system is contained within a Crystal Sphere, which is a physical object twice as large in circumference as the distance from the sun to the furthest orbital body. This space inside the crystal sphere is called Wildspace. Wildspace behaves much like what we know as space; it is a vacuum, and can be cold, but due to the crystal spheres, often it can be somewhat warm. Every crystal sphere is a little different.

Wildspace and Crystal Spheres are widely varied in characteristics; some are simple spheres, some have giants roaming the spheres, some have a nice hot sun at the center, some don't. In spheres where there is a sun, the temperature of space is usually warm and comfortable; however it can be dreadfully hot depending on a lot of factors. Planets orbit their sun, which is also called Primary, categorizing it with the planets as celestial bodies. These orbits are varied in distance and sometimes trajectory, much like the planetary systems we are used to. The stars one sees when one looks up into the night sky are not other suns, as the ancients used to believe, but rather magical crystals embedded in the inside surface of the crystal sphere. Some of them have obvious functions, such as operating the gates to get in or out of the crystal sphere. Others, who knows?

Surrounding all the Crystal Spheres is a thick fluidic matter called the Phlogiston.

The Phlogiston surrounds and connects all Crystal Spheres. Phlogiston cannot exist inside a Crystal Sphere, probably due to its flammable nature. Phlogiston can be ignited by as little as a lit candle, and the resultant conflagration can spread to many hundreds of miles around the center. Everyone knows if you travel through the Phlogiston you do NOT use anything with fire unless you have it contained. The Phlogiston has a planar attitude, as it is several times as thick at its center as it is above. The Crystal Spheres are described as bobbing in the Phlogiston, and in fact it is often referred to as a sea. Plogiston that exists above this plane of density, the atmosphere of space, if you will, is the same material as that below, but much thinner. The atmospheric Phlogiston reacts to the flow rivers with a similar movement, creating currents which can be utilized by sails and rigging to steer and drive spelljamming ships. The planar nature of the Phlogiston suggests a gravitic source somewhere below it, but nobody has been able to document such a source.

If you wish to travel from one Crystal sphere to another, say from Oerth to Toril, you will enter and navigate in the Phlogiston. The Phlogiston forms many flows, sometimes called rivers, that accelerate the movement of any ship inside it by a factor no one has been able to determine; speed is impossible to determine as there are no markers to use as a guide; one simply knows roughly how many days it will take to get from one sphere to another. Travel outside of the flows is possible, but much slower; travel inside Wildspace normally reaches about 4,000,000 miles a day, as an average. This speed varies according to the engine design of the particular spelljammer ship. In the Phlogiston outside of the flows, speed slows to just about double this speed, a fraction of the speed inside a flow. Also, the non flow areas tend to be inhabited by space born creatures, or hide outs for criminals. It's dangerous and slow, but if you don't want to be followed or detected in the Phlogiston, going outside the flows is a good idea.

The Phlogiston was assumed to be the end of it when men and other demi humans started to explore. It extends far above and below the spheres, so the assumption was that the Phlogiston was the universe, holding all the crystal spheres inside it. What they found out however, after enough exploration, was that there was an even bigger area of space, they called Void Space. They now understand their universe is a completely dark, starless, frigid void. There is no idea if the Void has limits, or where the Phlogiston is in relation to the "limits" of the Void; perhaps they never will.

The only recorded properties of Void Space is that it is entirely dark, appears to be entirely empty, and is as frigid cold as one will ever experience. Travel in Void Space is extremely dangerous, as the cold will eventually affect the oxygen inside your ship, and you will freeze to death. There is nothing in the Void, as far as anyone knows, so there's nowhere to gain supplies of any kind. Navigating in the Void is obviously dangerous and essentially impossible. For a while, if you look down you will see the pinkish glow of the Phlogiston, but eventually even that light gets lost, and you have no compass point to guide you at all. It is surmised that there have been many a ship go lost in the Void, but no one has recorded a successful discovery mission thus far.

Since there is gravity acting on the Phlogiston, and any ships which sail upon it, this means the gravity will also act on the interiors of the ships; therefore, gravity is usually not a problem. However, things in the Phlogiston have a way of surprising you, and the gravity that you have come to expect may suddenly erupt in the opposite direction or switch polarity and thrust the ship upwards. This is why most spelljamming ships have a gravity system of some kind.

Two major problems may become apparent to would-be spacefarers who are leaving their planets for the first time. Many spelljammers are completely open, their decks open to the airless void of wildspace. How does one breathe in wildspace and the phlogiston? Once that problem has been crossed, how does one ensure they don’t simply drift off their ship?

When any object or creature leaves a planet or large mass, they bring with them a bit of atmosphere that is proportional to their size. A medium-sized creature, like a human, brings a small envelope of air that is enough to last them for several minutes, but since they have no way of purifying that air, will then shortly asphyxiate, thus ending their journey into wildspace rather quickly. Larger objects and creatures, like a spelljammer, have a larger envelope of atmosphere they bring with them, allowing creatures to survive longer on them. For every ton of cargo space a spelljammer has, one creature has enough air for four to eight months, which means a spelljammer with a crew of 30 people should be at least capable of carrying 30 tons of cargo to have enough air for everyone on board for a multi-month long journey, like traveling from one sphere to another in the phlogiston.

In addition, gravity accompanies any suitably large object based on the object’s longest axis. Since spelljammers are longer than they are tall or wide, their longest axis is known as their keel and gravity exists along the center of that axis. Along this central axis, gravity pulls creatures toward it with the same force that would be on a typical planet. This does have the strange effect that if creatures are on the bottom of the ship, they are pulled to the center of their ship, so they can stand along the hull of the ship comfortably and can walk around the bottom of the ship to the top deck, the gravity keeping their feet pointing towards the central axis of the ship the entire time. Though, this has the strange effect that gravity can disappear when a smaller object drifts toward a larger object, like a ship coming in to dock with an asteroid. This means that the ship must orient itself properly to the asteroid, or find it suddenly crashing out of control of the pilot. This is also used in ship-to-ship combat as a smaller vessel can enter the other vehicle’s space and crash down on the larger ship, sometimes this is done on purpose by the smaller ship and other times it is done on purpose by the larger ship.

A Crystal Sphere, also known as a Crystal Shell, is an object of great size that surrounds an entire planetary system and forms a boundary that separates Wildspace from the Phlogiston. The size of Crystal Spheres is related to the size of the planetary system within them. As the name 'Crystal Sphere' implies, the objects are spherical. Although they are solid, Crystal Spheres have no gravity and no atmosphere. The material of which Crystal Spheres are made is an unbreakable dark ceramic. No magic is powerful enough to damage Crystal Spheres.

Travel Through a Crystal Sphere
Only a few methods are known to allow travel from one side of a Crystal Sphere to the other side.

Teleport or Dimension Door Spells
These spells (or magical devices that duplicate them) allow spellcasters to jump from one side of the Sphere to the other without physically crossing the Sphere.
Phase Door Spell
This spell (or a magical item that duplicates it) makes part of the Crystal Sphere immaterial and allows a spelljamming ship to pass through.
Naturally Occurring Portals
Most Crystal Spheres have natural Portals that pierce the Shell at various locations. These Portals usually appear and vanish at random times and are located in random places. Looking for these Portals can take a long time.
Stars
In some Crystal Spheres, Stars are located on the inside of the Crystal Shell. A few of these Stars are themselves also natural Portals through the Crystal Sphere. A spelljamming ship diving into the heart of the Star would pass through the Shell and appear in the Phlogiston. However, a high number of Stars are Portals to the Plane of Radiance or the Plane of Fire. Flying a ship into one of those Stars would be fatal.
The Spelljammer and creatures like Leviathans
The legendary ship called The Spelljammer and some large space creatures like Leviathans, seem to have an innate ability to create Portals similar to the natural Portals. These Portals close slowly over time, so ships can sometimes use them.
The Create Portal Spell
This spell duplicates the natural ability of The Spelljammer and Leviathans, but the Portals created by this spell have a much shorter duration. Portals created via this spell last no longer than 120 minutes.

Individual Crystal Spheres
The size of Crystal Spheres is usually determined by the size of the planetary system within them. Each Sphere is usually twice the diameter as the outermost planet within the Crystal Sphere, but Spheres can be larger than this. There are often astronomical objects that defy this 'double diameter' rule and orbit outside the outermost planet. Sages who support the 'double diameter' rule usually argue that these celestial bodies are additional astronomicals rather than true planets.

Nested Spheres
It is possible for a Crystal Sphere to contain another Crystal Sphere, or a series of smaller and smaller Crystal Spheres.

Main Theories
There are six main theories about the origin of the Crystal Spheres.

The Gods Created the Shells and the Flow
The gods created the Spheres and the Phlogiston in order to divide up the Universe. The gods split the Universe into Chaos (the Flow) and Order (the Spheres). They then divided up the individual Spheres among themselves in order to test Order against Chaos and Good against Evil.
The Gods Have Their Own Gods
These gods-of-gods created the Universe, and then placed the gods into the Universe. This theory gained more popularity in Realmspace in recent times.
The Phlogiston is a Naturally Occurring Object
The Phlogiston was there before the gods. The gods built the Crystal Spheres to settle and domesticate the Flow. The Spheres are like cities built along a stream and eventually they will grow and join together. When this happens the Phlogiston will cease to exist.
The Gods Fear the Flow
The gods fear the Phlogiston Flow and built the Spheres to keep it out.
The Gods Fear Men and Other Sentient Beings
The gods fear Humans and other sentient creatures and built the Crystal Spheres to keep them in.
The Phlogiston and the Shells are Naturally Occurring
Someone or something created the entire Universe and the gods moved in and took over. There is a secondary implication, that the gods have lied about creating the Universe to deceive mortals.