I brainstormed a little about Halos functionality and the moleculare effects of a array-activation and came up with this:
Prelude: Halo emmits radiation and kills organisms with ‘higher’ nervous systems.
Biology, radiation and molecules:
Since all cells are formed of the same compartments, not deviding animal- and plantcells, it either destroys a group of molecules that are in every cell, or it destroys DNA, wich is in every cell as well. Plant, Animal, bacteria or archeas, even Viruses would be effected. Archeas,m got other membrane structures to sustain heat, but the molecule base structure is the same.
Point is, on this level, the Endless immunity can not be explained by scientific means, assuming they are organisms based on cells, and the Halos use radiation to destroy (nervous) cells/systems.
The Endless could be artificial (not my idea, a guy commented on YT about that) but that would have been either noticed by the Forerunners or be on a level that would recreate basic cellular structures → and therefore have simular weaknesses to radiation or again, beeing easily detectable because it differs so hugely from other species.
Any artifical nervous system-substitution I could think of, would easily be detected by the Forerunners (e.g. metal veigns instead of cell-cell chains).
Summing it up: Either they are cellular based, and effected by Halo just as any other species (proven to be wrong) or they are (partial)artificial, wich would not require great research affords. - Assuming: Halo targets the molecules of nervous systems/their cells
[So I came up with something completely different. Here are my thought, at least the main steps of them:]
What if: The ‘scientific’ basis was never, that Halos effects nervouscells/systems but the Domain -
Forming the connection:
The Domain is of Precurser origin and the connection between life in the galaxy.
How does that ‘connection’ work → maybe it is just that, a litteral connetion.
Assuming, higher nervous systems emmit and percive (I hope this means detection of something) radiation (with an undefined structur that just does that bcs sci-fi-logix). This radiation and its intercommunication forming: The Domain.
These specieses could develope a dependance on it. E.g. like a remote control, or higher intend, as e.g. the one of the Precursers or the structure itself, as a selfsustaining mindlike structure. The Domain could be, what we call ‘instincts’. Many things are possible here.
[As I imagine it:] The Domain might function as a Brain itself, formed of countless individuals, whom are the centient beeings of the milky way. (just like a brain² or brain³)
Entities forming such a bound (could!) be linked in them originate from the Precursers initial creation/seeding of the milky way galaxy. - Humans, Forerunner [and so on… You get the point.]
Cutting the connection:
Now, if Halo targets the communication pathways, which form the Domain, Halo (might) eradicate entities linked/depending to it.
[How could this work?]
It might function with the same radiation that Halos emmit.
If the theorie on the interconnection via radiation is true, Halo could just function as a giant EMP for the Domain, blocking the communication of the individuals. A communications overload, just instead of an electron overload, as EMPs use, it fires another type of radiation.
Entities forming higher Nervous systems, and therefore link to/form the Domain would be effected, but it would allow ‘lesser’ beeings and plants to survive.
Now to the Endless. Their immunity to Halo might just be a reaction to ‘not forming a bond with’ the Domain. Not beeing dependent to the only thing effected by Halo, the Domain, makes them immune to it.
[What came before:
Now turning the back to its functionality and head on to its position in the grand sceme/the plot of the Halo universe: The Mental and its heritge.]
Seeking Knowledge, Seeking Control:
It was always about responsibility and the Mental.
The Precursers knew, the Foreunners would not seek to understand, but control what they see. After all they created them, they could predict qhat they might do, they were their parents. They knew they [Fs] would not seek knowledge what the Domain and Mental truely represented. But the Humans seek knowledge, to understand things for their inherent beauty, not (just) to control them.
Taking and executing Control
Following the idea ‘you can not take responsibility for sometuing you do not understand’. They denied the Mental to the Foreunners and enraged them, leading to war.
Since the Domain was simply taken over by the Forerunners, and taken for granted and something they can fully control, they (might) not have seen its influence or linkage on all beeings (depending on my theorie on Halos function). Since they might have thought they understood it well enouth, it did not bother them.
They believed the Mental was just biodiversity and controlling what exists, rather than letting tge things controll themselfs. So they never seeked the perspective needed to truely understand its structure and therefore (blinded by their ignorance) where not worthy of taking over the Mental. (Im not well enouth informed on how the foreunners saw the mental, but the point is that they did not understand the Mental and did not seek to do so)
How did we get here? Ah right, the Endless are Immune to Halo.
Here ends my theory-prelude, leading into, why the Endless are immune to Halo [Note: Im a little tired and want to keep this short, so sorry for cutting it of/just listing things]
A teachers path - A Precurser line:
They knew what the flood-infestation of the galaxy would leed to, including Halo. To show the Humans their place, they [Hs] need to understand the Mental (and Domain). The Endless are unlinked to the Domain. They are static, never changing, untied to evolution and diversity in their own kind. They might be a tool to get Humanity on track, to find out, what the Mental an Domain reals are. A simple didatic tool: “here, they are not dying, think about out why they are immune and you will see”
The true Reclaimers - another Precurser line:
They predicted or postulated, that only a species outside of the Mental [outside the Box] could take real responsibility, since they are not biased by the system they shall superwise. Therefore, they unliked the Proto-Endless from the Domain
Unlinking ‘Creation’ - evolutions line:
It was pure coincidence, that the species ‘lost’ a connetion to the Domain via Evolution
A Stranger in my garden - an independant one :
They originated extragalactic or came from a part of the milky way galaxy that was not occupied by Precursers or Forerunners. - Both of these would leave out the question, why no others followed them after the Foreunner died out.
And here I get a little too tired. Let me know your toughts on this, after all, its just theorycraft with little to no evidence
Merry Christmas to you all.