Update: I have figured out how to get more dark aura creatures!!!
Turns out I wasn't far-off with my "limit on potential spawning" theory two weeks back. If you run away from an enemy and it vanishes, the game still holds that enemy in a kind of stasis storage but - and here's the part that's strange - the enemy only "exists" somewhere near the spot it first appeared. If you exceed the maximum amount of stored enemies, the game will stop spawning new ones, leaving you free to run around (but ultimately dark aura-less).
Unfortunately, in order to solve this issue, I had to open the debug menu using the "`"/"~" key. In that menu, you can see how many enemies are currently stored in quantum space, ready to appear the next time a new space is uncovered. The enemy that will appear is limited to whatever biome you're in (or nearby), and since each of the lil desert sprite thingies count as their own enemy, they fill up the enemy total real quick. If your home base is near a large stretch of desert/wasteland biome, consider venturing into that area more to do some spring cleaning, and lowering the total enemies count in the world. You'll know you're doing it right if at random points in the game the total monster count goes up by one, which means you're about to see a new monster!
Once you have new monsters spawning, I advise moving to the patch of void that is most convenient to you, and do the usual quantum pacing strategy. Sometimes you will get the other ghosts/spirit/thingies spawning, just remember to dispatch them as they arise. Rinse and repeat, and you should eventually find the dark aura creatures!
Happy hunting, everyone. This is Elfie, signing off! :D