The following is my head canon to make sense of the D&D multiverse.
The "alchemy" of the Forgotten Realms setting is moreorless the same as reallife physics. At least, alchemy describes physical properties in ways that modern physics can make sense of. (Where alchemy produces a potion that shapechanges a drinker into a frog, could be a theoretical nanobot technology, or just "magic".) Where alchemy describes normal phenomena, such as gravity, it corresponds to reallife physics.
Alchemy describes four elements, but these elements mean the four states of matter, rather than the 100+ atomic elements. Namely: solid (earth), liquid (water), gas (air), and plasma (fire).
Here the fiery plasma is the "heavenly fire", a distinct state that happens at the sun, stars, and lightning, where electrons and protons separate from each other in a somewhat stable way, resembling gas but with electromagnetic properties. The fire of a candle is merely hot incandescent gas, thus an aspect of alchemical air, yet its luminosity resembles the fiery plasma of the sun, which is alchemical fire. There is also a "dark fire", being the nonluminous cold plasma, such as the plasmosphere around the planet Earth, beyond the atmosphere between the Earth and the Moon. Either way, alchemical fire corresponds to reallife plasma.
The D&D damage types relating to plasma − namely the heavenly fire, elemental fire, or alchemical fire − are: hot fire, magnetic lightning, and nuclear radiant. (Even so, I prefer to pair lightning damage with thunder damage, thus mainly associate lightning with the air element thematically.)
The four elements correspond to recognizable reallife physics.
The fifth element called "ether", sometimes called "spirit" (in the sense of a substance that spirits are made out of), is unlike the other four elements because it is immaterial even tho it is physical. The ethereal plane is the plane of the fifth element, ether. Ether is force.
The reallife fundamental forces are gravity, electromagnetism, and the nuclear forces. D&D telekinesis is the manipulation of ethereal force. In the same way that reallife matter is made out of nuclear forces (thus in a sense matter is ultimately immaterial), the four elements are made out ether. Matter is an involution of force, turning in on itself, forming a unit with material qualities.
The ethereal plane, that is the level of existence that is force, overlaps the material plane, that is the level of existence of matter. The part of the ethereal plane that overlaps the material plane is called the "shallow" ethereal plane, or the border ethereal plane. There are also parts of the ethereal plane that have little or nothing to do with material plane. These parts beyond the material are the deep ethereal plane. Instead of bordering the material plane, the deep ether borders the astral plane. Thus there is a continuum between astral thought forming ethereal force and eventually, by means of force, forming material matter. Thought creates matter, via force. The deep ethereal plane is where the "spirit worlds" operate separately away from the material world.
In my view, both the feywild and the shadowfell, are spirit worlds that are aspects of the ethereal plane. Albeit, the influence of positivity and negativity, respectively, qualitatively alters them. Likewise, both the fey and the shadow comprise a border realm that overlaps the material plane, where one can view and interact with it. The border shadow engages the material plane thru the distortions of negativity, so that the matter appears gloomy and decaying. Among matter the shadow creatures are ghostlike. Oppositely the border fey engages the material plane thru the distortions of positivity, so that the matter appears full of life and visually vibrant. Among matter, fey spirits exhibit heightened beauty and an enchanting psychological charm. The deep fey includes areas beyond the material world, such as delight domains where each manifests the influence of a community of fey or even a single. Oppositely, the deep shadow includes the dread domains.
The elemental planes are actually aspects of the deep ethereal plane, being the forms of matter, but not the substance itself. The elemental beings there are forces that behave like matter. These elements are the building blocks that the material universe will be made out of. Note, I also view the True Neutral astral dominion, called the Outlands among other names, to be the same thing as the ethereal Elemental Chaos. Either the Outlands itself is also ethereal, or else the Outlands and the Elemental Chaos closely border each other.
Notably, the border between the ethereal and the material is where the ethereal plane is actually part of the material plane. For example, gravity is a normal part of the qualities of matter in the material plane. Gravity is an ethereal force that exists simultaneously in both the ethereal plane and the material plane. If one manipulates gravity from either the material or the ethereal, it likewise alters the gravity of the other plane, because it is the same gravitational force. Likewise, telekinesis from one plane can affect the other plane. Force damage from one plane can affect the other plane.
When a mage "conjures" an other otherworldly creature from a nonmaterial plane, the magic forms a "conjuration" − namely a force construct made out of ether − that can serve as an avatar for that otherworldly creature. If this avatar within the material plane gets destroyed, the otherworldly creature remains safe elsewhere, albeit experiencing the "death" of its avatar can be unpleasant or traumatic. Similarly, the "magical energy" of a conjuration or an Eldritch Blast force damage spell, are made out of ethereal force.
It is impossible for finite beings to exist in either the positivity plane and the negativity plane. These planes are beyond the fabric of spacetime. The positivity is the infinite energy. All thoughts, forces, and matters, are made out of positivity. But these are finite packets of positivity able to exist, move, and change because of the fabric of spacetime. Oppositely, the negativity is absolute nothingness, without any finite quality. It is like an infinite black hole that only the infinite positivity can satisfy. But the separation between the positivity and the negativity is precisely what allows the fabric of spacetime to come into existence in the first place. The negativity is nothing, and lacks energy in itself, but when existing things drift toward it − toward annihilation, disintegration, decay, death, oblivion − its "influence" is improperly, or out of convenience, viewed as a kind of "negative energy". There is more about how positivity and negativity interact, but now to focus on the Spelljammer astral setting.
At this point, the black hole in Doomspace is a region of extreme gravitational force that annihilates matter. By definition, this is an area of extreme ethereal force. But its nihilism implies interactivity with the negativity plane. Perhaps there is a "rupture" in the ethereal plane where a single point, namely an "eye", that reveals the absolute nothingness of the negativity plane. The eye of negativity might resemble a Sphere of Annihilation but with extreme gravity pulling toward it. The association of both the ether and the nothingness might imply an ever-expanding shadowfell region that orbits around the negativity rupture, but within the event horizon of the black hole.
