Supersphere Theory

To understand the theory, you must understand something of Superstring Theory. In short:

• All particles are, ultimately, vibrating strings.
• The strings are as much smaller than electrons as electrons are smaller than, say, golf balls. Maybe not quite. Who cares.
• The exact vibration of the string determines the nature of the particle--is it an electron? A proton? Technically, the strings are quarks, not hadrons, but you get the idea.
• Strings are also at the heart of Bosons (particles which transmit force, such as photons, gluons, and gravitons).
• The strings are anchored at both ends to a 10-dimensional firm surface. That surface is known as a "brane".
• Our entire universe is one brane.
• There are many (possibly infinite) branes, though to be in parallel.
• The branes themselves can vibrate, occasionally (as in once every 10-50 billion years or so) colliding, causing a massive release of energy, aka The Big Bang. Maybe.
• The branes are all 10-dimensional: 4 dimensions (3 of space, 1 of time) we recognize, and 6 more we don't. We're all 10-dimensional. It's just that the final 6 all collapsed to an infinitely small space. It might be more than 10. Whatever.
• Gravity is transmitted through gravitons, whose strings, for whatever reason, are not firmly anchored to any brane. Instead, they are closed loops. When a gravitational force is generated on one brane, the gravitons radiate in every direction. Only an infinitesimal fraction remain in the originating brane...thus explaining the incredible weakness of gravity compared to the other 3 fundamental forces. The remaining gravitons enter other branes, effectively transferring the force of gravity from one universe to many others. This is the leading theory behind dark matter.

Now that you've mastered string theory, consider this: one might become able to transit between parallel universes by somehow uncoupling ones strings from one's home brane, becoming, like gravitons, a substance which can float freely between them all.

How would you do this? Who knows. It's hyper-science, probably a million years in our future, if it's even possible at all. How would you keep yourself together? Again, who the fuck knows. You'd probably just encode all information necessary to describe "you" into a single super-particle, and let the rest of you just dissolve into meaningless dust. How you'd reassemble yourself on the other side...who knows. At least you have all of your quantum data at hand when you get there. Of course, you'd have to re-couple yourself to the new universe, and be ready for a possibly different set of physics than you're used to.

It's unthinkable that humankind could develop the technology to do this in the modern day. However, it's not entirely unimaginable that an ancient civilization, one who developed perhaps hundreds of millions of years before us, learned to do exactly that. They explored the multiverse, and, on some worlds, they left artifacts. Maybe they were lonely, and they wanted company. Who knows.

It may be possible for us primitive humans to activate such artifacts, and use the same technology on ourselves. That was the case with Paradox, Inc., who found such a device in a deep archaeological survey of Antarctica in 1979, and began, with vast government funding, to research it in great detail in a secret facility near Sierra Vista, NM. One of their own, Hyacinth Knight, learned too much about their true motives, and set out to warn the people, and the President, but was eliminated before she could do so.

Or was she? In fact, the weapon they thought would annihilate her from their universe was actually a method of transportation. In demonstrating the above theory, still not even remotely understood by Paradox, her quantum information was uncoupled from her universe and sent on a long journey through the branes. Ultimately, she arrived in Abeir-Toril, partially merged with the essence of a nightmare shard (see The Nightmare Crisis), and from there, preceded to unravel a vast conspiracy that threatened the fabric of reality across all universes.

So, while the theory is still light-years beyond us, we know, at least, it works.

The primitive screwheads of Abeir-Toril don't have the first clue about physics, let alone superstring theory. However, through various magical means, they have divined a surprising amount of information about the nature of the multiverse. Despite lacking a basic understanding of atomic structure, let alone the subatomic reality of strings, they have developed "magic spells" which can duplicate the most advanced science known to uber-beings of our own universe.

Every teleportation spell, every plane shift, every gate--even summoning spells--uses the same method to traverse parallel universes as the unknown alien race who pioneered the technology in Earth's past. Such spells even utilize the same energy source, known to humans as Dark Energy, which is ultimately the baseline energy of the cosmos itself, the sea in which the branes of reality float. It is thought to be undefinably infinite.

Creation

The Realms exists in a universe with an entirely different structure than Earth's. Every star and world in it was created by an unknown progenitor race, possibly consisting of a single being, likely from another universe. It could be that the Realms' universe was a "blank slate", a brane unpopulated by any matter or energy until its creator arrived.

The "Sticky" Nature

In the beginning, it is thought, the creator(s) brought forth the suns and the worlds. There are billions of worlds, held in a static space--gravity does not cause them to contract upon each other. The brane itself was somehow modified to restrict the force of gravity, likely to enable said static universe. It is possible this property allowed Hyacinth Knight to land on this brane much as a fly lands on fly paper, or a particle of dust is attracted to, and clings to, a surface with a static charge.

Structure

The galaxy-universe of the Realms is organized in a rough sphere, with a higher concentration of worlds near the center. It seems the creator(s) grew more experimental as they ventured further from the center; stranger worlds and lifeforms abound near the galaxy-universe's edge. Beyond what is considered the "edge" of space, there are few known worlds--it is mostly darkness. An unknown force shields all means of detection beyond the outer rim--telescopes reveal nothing--but it is known that one can physical traverse into that space, observe it, and return to tell the tale. However, all who have done so have suffered horrible fates, and have explained very little in adequate detail before doing so.

The Outland

The population of the worlds of the Realms universe is not entirely home-grown. Its "sticky" nature has caused many brane-jumping beings to find themselves within it. The greatest of these is known as The Outland, a massive, wheel-shaped universe with strikingly different physics than that of the Realms. Beings from this universe often project into the Realms cosmology, masquerading as "gods" and demanding the "souls" of worshippers. In reality, they are transferring the quantum information of sentient beings to their own universe, a process which strengthens their own power, due to the laws of physics within that universe.

The Shadow and the Ethereal

The Realms is sandwiched between two parallel universes known as the Shadow and the Ethereal. The former appears to be a dark mirror of the Realms universe, mostly devoid of life, and reflecting a strange temporal delay. It did not fracture off at a particular time, but instead diverges at all points in time, but through a veil of temporal static. Its peculiar origins have been the lifelong study of many planar cosmologists.

The Ethereal Plane is a region of fluidic space, which picks up "imprints" of the reality it so closely borders. The net effect of this is that "ghosts" of living beings can form due to particularly strong psychic impressions, as well as vague imprints of physical reality. Denizens of the Realms often enter this realm to travel through their own, ignoring physical barriers in their reality, while still able to navigate by following the visual imprint of their home world. The Ethereal Plane's brane is congruent on one surface with the Realms brane, and thus the distance between two points on one brane which touch two points on the other brane is the same in both universes. Beyond this surface, the Ethereal Plane universe becomes ever more unusual.

The Astral Plane

The denizens of the Realms universe often refer to a universe known as "The Astral Plane". This is, in fact, not a universe at all, but the space between universes. When uncoupled from their physical reality, the thought-forms of sentient beings can traverse this universe at the speed of thought. They can explore the vastness of the multiverse, seeing the thought-forms and meta information each universe projects beyond its own existence. However, they cannot physically enter any other universes in this way, except two: their own, and The Outland. Their own because of its unique "sticky" property, and the Outland because of its unique physics.

The Crystal Spheres

Ruins of ancient civilizations millions of years older than humanity's on Toril suggest that, in the beginning, the Realms universe was without crystal spheres. Many have speculated on why this may be.

The leading theory has been that the spheres were constructed after the fact by the Creator(s), who wished to contain the advance of the denizens of Outland, who were increasingly invading their domain through connections to sentient beings living on their worlds. This theory is substantiated primarily by the efficacy of the spheres in achieving exactly that; no extraplanar (extra-branar?) entity is able to violate a crystal sphere.

However, it can be argued that the Creator(s) ought to have a more effective means of banishing the invaders. After all, the sphere approach sequesters entire worlds, dooming them to the alien infestation, and does not necessarily prevent Outland denizens from making their way through the Astral to other worlds.

It is notable, though, that aliens could only do so by contacting the minds of sentient beings, and said beings would have had to realize their thought forms before allowing said contact. In other words, only sentient beings who transcend their home world could allow Outlanders to bypass the spheres and arrive upon a new world, through their minds.

The spheres may originally have been intended to prevent the proliferation of said sentient beings. They appear to space travelers as just that--incredibly massive spheres of crystal glass, impenetrable by all known forces. However, the spacefaring races of the Realms universe have long since learned how to bypass the spheres, by using the very same kind of magic they use in other conjurations to form a dimensional shortcut around the obstacle.

In any case, the spheres do exist, and they do effectively block the influence and travel of Outland denizens between them, although they can frequent multiple spheres, if the local residents have knowledge of their existence, or, in other words, "believe" in them.