The hypothesis of the living Aetherius and Oblivion

In the beginning, the Godhead dreamed two infinite, amoeba-like beings, Anu and Padomay, loosely based on the Godheads life before his ascension. These beings were almost mindless, yet each had one primal desire. Anu wanted to absorb Padomay into itself, Padomay wanted to sunder Anu into many tiny pieces. Anu managed to suck a piece, a bubble out of Padomay, which was Sithis. Padomay then managed to split a piece out of Anu, which was Anui-El. Those two smaller beings were slightly less mindless than Anu and Padomay, but like them, they tried to absorb/sunder each other. Anui-El encased Sithis in order to eat it, but Sithis pushed itself against it in order to rapture Anuiel from the inside. Due to all of Sithis' creatia pushing itself from its inside to its outside in order to put Anui-El under pressure, the density of its center decreased until it became something that was neither Anui-El nor Sithis but pristine nothingness.
As both of them were equally stong, they were soon locked in a stalemate in which both were unable to move.

Anui-El was now known as Aetherius, a giant pool of magicka which is light, a spherical shell containing Oblivion.
Sithis was now known as Oblivion, a giant pool of chaotic creatia which is azure plasm, a spherical shell containing the grey maybe.
The grey maybe did not have a proper name for it was absolutely nothing.

At the fringe between these forces, Oblivion ripped out tiny pieces of Aetherius and Aetherius sucked out tiny pieces of Oblivion. The pieces made out of magicka became the Anuic et'ada, the pieces of Sithis became the Anuic et'ada. The first two et'ada were Aka and Lorkhan, who drew the grids of space and time onto the Aurbis.
These being inherited the primal Anuic/Padomaic urges. However, some beings were created from a balanced mix between Anuic and Padomaic creatia. Those being were the Aedra, who did not share these urges since they cancelled each other out.

Most Padomaic et'ada went to Oblivion, where the strongest of them shaped the azure plasm into matter and into their laws and created their empires.
Most Anuic et'ada went to Aetherius, where most of them enjoyed being amorphous, although some created realms out of magicka like the Daedra did in Oblivion.

Lorkhan left the Aurbis and he saw its structure and decided that he wanted to leave not just the Aurbis but also the Dream. He realized that the Anuic and Padomaic primal urges would cloud ones mind too much to archieve Amaranth, so he wanted to create a place where et'ada would be reduced to truely neutral beings.
He realized that what the Daedra were doing was not creation but mere transmutation, so he wanted to build something in the only place where true creation was possible: The grey maybe.
He realized that in order to grow, you have to be trimmed first.
He realized that in order for everyone to succeed, everyone who doesn't must die and start anew.
He realized that nobody would agree to this plan, unless he lied to them.

He convinced most et'ada to participate in the Mundus project, but the Daedra did not join, for they were content with their imperfections. They crafted many concept and built Nirn out of them, but secretly Lorkhan added a very special and entirely new concept: Mortality. As the et'ada felt their power fading, some started to flee. Magnus was the first and the most powerful of them. In order to escape, the Magna Ge ripped many wounds into Oblivion, which could not heal as magicka from Aetherius flooded into them and became Auri-Els many tentacles. These tentacles are the stars and the sun and through them magicka which is light bleeds from Aetherius.

Some et'ada were powerful enough to escape, but they didn't, as they saw creation and were content with it. They flew into the grey maybe in order to orbit Nirn as the eight planets.
Some et'ada took ethereal forms and became the earthbones, whose will are the laws of physics.
Some et'ada could not escape death at all, but they lived long and had many children. Each generation was more mortal than the last. The Aedra became the Ehlofney and Hist and the Ehlofney became men, mer and everything else.
Before the earthbound Aedra died, they managed to kill Lorkhan and they trapped his soul inside his heart.

TL;DR version: Sithis is Oblivion, Anui-El is Aetherius, Mundus is neither.