It seems far too much like aether, i.e. something made up to fill a gap in knowledge without much evidence backing it up. "Look, my equations don't work out in every situation. EUREKA! If I just make some shit up like say, invisible matter that doesn't interact with other matter except through gravity, I can make my equations work!".
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It must be that the model needs additional generalization rather than inventing magic stuff.
You've heard of the Einstein quote “Everything should be made as simple as possible, but no simpler.” - have you ever thought that your statement may also make stuff 'simpler' than it is?
For long in human history, it was posed that the earth was flat - until someone 'made up some shit' called gravity that allows us living on a ball shaped earth, without us sliding off the side or making people in Oz simply fall off the planet...
How about hydrogen? For long in history, hydrogen wasn't known to us - until someone 'made up some shit' called atoms and molecules -- in the process picking the name 'atom' (something indivisible), until some other shit was made up (protons, neutrons, electrons), which is the smallest that exists -- well, apart from some other shit stuff that was subsequently made up (quarks).
Obviously, some of this stuff was 'discovered' first, and not made up - but all of it would simply have been some 'invented some shit' had someone just posited its existence before.
Many of Einstein's ideas (frame dragging, time dilation) might still fall (partially) into the same category - it's something that must exist for some formula to work.
Note - this is not saying all of the above would be fictitious, but saying that sometimes you need to posit things that you can't observe, so you have something you can go and look for and finally discover and prove, or simply disprove.
Superstring theory is not yet proven, yet it's a useful concept to talk about among people trying to prove its existence, and it allows making the whole 'matter' thing even simpler -- yet, it may still fall apart. Or - maybe there already is a new theory for it or something else.
Just discrediting any hypothesis as 'making shit up' is stupid, and not helpful to any science. Discrediting making up a theory (and for it to work some as of yet unobservable force/matter), sounds more like religious dogma to me. Sure, many theories turn out to be wrong, but positing a theory (and all it requires) allows people to talk about and test an idea, and may give other people ideas on how some of it could be tested in order to prove or disprove it.
And another point - if dark matter / dark energy existed, it would make some models and some understanding simpler, than by introducing something else different.
In the end, we will see whether "dark energy", or "gravitational polarization of the quantum vacuum", or even both go the way of the "Phlogiston"...