We know that the Phazon meteor which struck and divided Aether was really a Leviathan. Though we never see it, it's presumably still somewhere on the planet - whether in Light or Dark Aether is unclear. But anyways, the Emperor Ing was said to be heavily mutated by Phazon, which can be found all around its boss chamber. It's possible that the Emperor Ing was chosen and corrupted by the Leviathan to be its guardian, similar to what happens in Corruption to the bosses like Mogenar and Omega Ridley.
- I want this to be true, but the Emperor Ing was not found inside of a meteor, unlike the Metroid Prime or any of the guardians in Corruption. It's possible that the Emperor Ing was feeding directly off the meteor's Phazon and that's why it mutated so quickly, but it's unlikely that it was directly chosen to live in and protect it. Of course, that does lead one to wonder if something else became the Leviathan's guardian...
- It could be that the Sky Temple itself was the Leviathan, or at least partially constructed from it. From the ground it looks like a giant rock. Interestingly, its layout is similar to the Great Temple in Light Aether but upside down for some reason, which is unique among the Dark Aether variant locations.
The Ing have a developed culture and civilisation that's delved into by the lore scans of each Ing and Darkling Samus encounters. They even have arenas for spectator sports, albeit mainly violent ones. It seems unlikely that the species simply popped into existence from nothing when the Phazon meteor struck Phaaze, so perhaps Dark Aether had always existed for as long as its light counterpart, but in an unreachable alternate dimension. Since the meteor vanished after impact and the largest concentration of Phazon is in Dark Aether, it's likely the impact crater is in Dark Aether, and consequently the corruption of all native flora and fauna is happening solely on the dark version of the planet. If the Ing weren't malevolently evil to begin with, they absolutely would be after having Phazon infesting their homeworld for a few decades. Furthermore, even if the Ing succeeded in getting their planet to completely usurp Aether, they would be doomed anyway since Phazon would eventually corrupt their entire species. Though the Ing may have been nicer before the advent or Phazon, killing them at the point the game starts is a mercy.
- To elaborate on this: the Ing may have also seen the Luminoth as invaders. What is the coincidence that Light Aether's temples and structures happen to be the exact structures present in Dark Aether with the same control modules and so forth? Most likely, the Luminoth's buildings suddenly appeared in Dark Aether, leading the Ing to believe they were being invaded first. They certainly don't seem to be intelligent enough to reason with the peaceful Luminoth and thus it culminated in war.
- The Ing are shown to be somewhat clever both in-game and in the backstory: they possess and corrupt machines, they manage to figure out how to drain energy from the Controllers and funnel it back into their own world, and they (correctly) identify Samus as a threat and attempt to neutralize her, and even put the suit upgrades they stole from her to good use. Even if they're not individually sapient, they're at least Bee People with a sapient leader (the Ing Emperor). It/they realized, just like the Luminoth did, that Aether couldn't survive with its planetary energy split in two, so they went about securing their portion of the world at the cost of "Light" Aether (again, the Luminoth also do this, and they're portrayed as the heroes). Samus decides to fight for the Luminoth for several reasons: her gunship is in Light Aether and would be destroyed if the Ing won; the Ing stole her suit upgrades, and she wants them back; and her own conscience wouldn't let her sit this one out, so she goes with the Luminoth because of Dark Is Evil / Light Is Good and What Measure Is a Non-Human?.
- In the same way that caterpillars become butterflies, phazon becomes the Ing, likely with other stages down the line. Phazon is The Corruption; the Ing have upgraded the much slower, cumulative effects of phazon corruption into full-on Demonic Possession. Phazon mutation tends to make things bigger and/or spikier; Ing possession does much the same and default Ing tend to have plenty of spikes of their own. And correct me if I'm wrong, but the Luminoth have no documented history of anything like the Ing before the meteor which spawned them. I'm reminded of a line from Evolution (2001): "The organisms were dormant in space. They hit Earth's atmosphere and bam! Heat. Fire. That's the catalyst." Likewise, that heat provided the energy for the phazon to grow into the Ing (maybe with a bit of dimensional flux mumbo-jumbo mutation on the side, explaining why they're purple instead of blue). Finally, consider the Phaz-Ings you can encounter on the GFS Valhalla in Corruption, which are pretty much identical to the Inglets. No doubt it's more than a lazy Palette Swap.
The Ing's defining ability is being able to cross dimensions at will in order to possess creatures and transform them into Darklings. They don't need an active portal, but can't survive in the light dimension unless they can possess something. Dark Samus demonstrates her ability to copy the attacks of the Hunters in Metroid Prime 3, so perhaps she managed to survive the destruction of Dark Aether by copying the Ing's power to cross dimensions. Dark Samus may even have learned how to possess and control the Space Pirates and Hunters in Prime 3 by copying the Ing's methods to create Darklings.
A few messages throughout Echoes leads to the conclusion there's only one Energy Transfer Module. This module was somehow lost to the Ing, which then used it to steal the energy from the three temples. Since the Ing have to possess living creatures to remain in the light world, they would have to rely on local creatures to possess, hence giving it to an Alpha Splinter to try and assault U-Mos' lair and steal the final energy left in Aether. This, of course, doesn't work as Samus gets it first. Hence we get a logical reason why the Energy Transfer Module was where it was.
Furthermore, it makes sense that they'd wait to storm the Great Temple. Splinters are weak, and U-Mos is no pushover from what we've learned. It's entirely possible U-Mos' hard-to-breach location and his skills held the Ing off for a considerable degree of time.
Although he was obviously not active, he could have managed his way into Aether/Dark Aether, stalking Samus far in the background at every turn (or he could just be watching from a screen while recovering from his wounds). This could also mean that Ridley and Dark Samus' alliance was mutual rather than forced, as Ridley would probably see interest in Dark Samus and would want her aid to destroy the bounty hunter forever. And her aid did he receive.
