The problem with dark matter is not that it is ad hoc. It is that it feels un-parsimonious to resolve a discrepancy in rotation curves by inventing five times more stuff than you had, all undetectable, and distributed just right to make all your curves come out right.
It turns out that the mass of neutrinos produced so far adds up to as much mass as all the rest of particulate matter. I guess neutrinos must be distributed about evenly throughout the universe. If we could not ever detect neutrinos, that would be awkward.
I have not heard of a mechanism by which these dark matter particles can cool and condense to clumps to seed galaxies. By contrast, baryonic matter gets to emit photons to give up kinetic energy.
Particle physics has a pretty good track record of inventing undetected particles to solve missing item problems, then finding that particle later. I'd guess we're up to hundreds of such particles like this so far, with really big finds being positrons (and all anti-partickes, needed when adding relativity to spin), neutrinos (needed to account for missing mass), all the quarks (needed for certain symmetries, among others), and the Higgs bison itself (needed to explain masses).
So don't discount adding currently undetected things as explanations for surprising measurements. It's been extremely fruitful.
Got no objections to inventing particles to take up and carry off whatever bit of stuff has to conserved: neutrinos, virtual particles, gluons, what-have-you.
I don't really even object to inventing five times the mass of the known universe.
It just feels, as I said, un-parsimonious. It feels quite a lot like inventing God to patch a "missing link" in your evolutionary succession. Maybe you'll find God there somewhere, but it doesn't seem like the first thing to try.
So, maybe dark matter really will turn out to be an ordinary axion or something detectable only if you manage to squint just right, and God just loved those so much that almost everything is them, and we are all made out of just leftover scraps. Maybe there are dozens of elementary axions, with relationships and exciting interactions we can never figure out because we can't touch them in any way but gravitationally.
I don’t think you’re giving enough credit to physicists.
Dark matter isn’t just adding a fudge/“God” parameter to the equations to make the numbers work. The “stuff” that’s thought to be missing is described with very precise properties… the fact that it’s “dark” means something, the fact that it’s “matter” means something.
Well, in this case they're not just inventing nonsense to fix one little thing. There are a lot of various pieces of evidence, all pointing to this fix. If you want to patch up a lot of little pieces of evidence, each with it's own magic, that would be even more "un-parsimonious".
The evidence for dark matter is pretty solid.
As this [1] article states, while giving 5 independent reasons scientists think dark matter exists, "No other idea explains even two of these".
Wikipedia lists eleven different places it shows up.
The research literature has more. And AFAIK there is no other unified (or even close to unified) explanation for all these observational pieces of evidence.
On the flip side, why would you expect us to be equipped to observe more than 20% of matter give the narrow range of our vision and hearing?
Anything that doesn't interact strongly at our scale wouldn't be useful evolutionarily.
It turns out that the mass of neutrinos produced so far adds up to as much mass as all the rest of particulate matter. I guess neutrinos must be distributed about evenly throughout the universe. If we could not ever detect neutrinos, that would be awkward.
I have not heard of a mechanism by which these dark matter particles can cool and condense to clumps to seed galaxies. By contrast, baryonic matter gets to emit photons to give up kinetic energy.