“Dark matter” always seemed like a cop out to this layperson. You can’t just give up and attribute problems with your theory to a mysterious substance that can’t be seen or measured!
Two early cases of this were that if invisible matter exists, there should be gravitational lensing around seemingly empty space, which was then found, and that if if something like dark matter existed before recombination, then a certain pattern should be found in the cosmic microwave background, and a few years after the prediction, sensitive enough equipment was deployed that could see enough detail in the CMB, and the prediction turned out correct.
Today there's something like a dozen such predictions that have come true, ranging in scale from kiloparsecs to gigaparsecs, from things happening today to things that happened in the early universe. When a theory makes accurate predictions across many problems and areas, that is good evidence for the theory.
"Laypersons" just never seem to know anything about dark matter, including the shitload of evidence from several different types of observations that there's something there that can't be explained by just tweaking some equations. Of course that never stops them from having an opinion based on a total strawman.
https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...
In particular we’ve observed some galaxies that seem to have little to no dark matter. Gravitationally, they behave the way that you would “naively” predict without the need to plug in a correction term that represents dark matter. This indicates that there is something physically different about these galaxies (like an unobserved substance), and it points away from MOND-style theories.
For what it's worth, I hate it too, and I'm quietly betting on the paper a while back that showed GR was all you need as long as you can be bothered to do the really really hard maths.
Models are quantitative entities – this is physics we're talking about. They almost always have quantitative parameters whose values initially have large uncertainties. We make observations in order to tighten the error bars of those parameters – to rule out parameter values inconsistent with observations.
Now, if we found that two observations are inconsistent with each other – each implies a parameterization incompatible with the other – and could rule out experimental error via repeat experiments etc, then that would be evidence against the entire model. In dark matter's case, we might realize, for example, that we cannot predict both galactic rotation curves and CMB anisotropies with a single DM model. That would be evidence against DM. (But that's not the case, both seem to be perfectly consistent with a single, well-constrained model.)
I'm saying (as others seem to also be saying) that it feels akin to saying "trust me, the model is right, we just haven't been able to find the missing $1 million but I'm sure it exists somewhere"
It's not a particularly innovative or insightful criticism of current models, but that's what it feels like from the outside looking in. Chalk it up to ignorance, if you'd like, but I still feel like asking "are you sure you can't fix the model instead of plugging it with some unobservable feature?"
It feels like TFA is arguing "the way we calculate interest in the current model is wrong, here's a new formula". The jury is still out on whether that new formula breaks other parts of the model, I suppose
Another analogy: we find a person, shot, in his home. There's a bullet inside the body, calibre and rifling marks point to a particular type of firearm, powder burn marks indicate a point-blank shot, all the CSI stuff. But the murder weapon is nowhere to be found. Do we assume that the weapon – and the perp – does indeed exist, or should we "tweak the model" to include a way for bullets to spontaneously materialize out of thin air and kill people?
> Chalk it up to ignorance, if you'd like, but I still feel like asking "are you sure you can't fix the model instead of plugging it with some unobservable feature?"
I don't understand how people think there's a distinction between "introducing a particle" and "fixing the model". Doing the former is doing the latter! Anyway, yes I do chalk it up to ignorance, because that's still a strawman, assuming some kind of a Scientific Orthodoxy that has decided that Dark Matter It Is, Claiming Otherwise Is Heresy.
It's tiring to repeat this in every thread about DM, but – everybody and their dog has been testing the dark matter hypothesis for several decades now, obviously, because finding definitive evidence against it could be worth a Nobel, and so would figuring out a way to detect dark matter particles in laboratorio. And thus far they've found zero contradictory evidence, but instead in several other subdisciplines of astronomy, observations have been made that just happen to fit exactly the same sort of dark matter model that was originally only devised to explain the galactic rotation curves, without any fiddling. All the new evidence has simply tightened some parameters of the model that used to be less well known.
Meanwhile, no competing hypothesis has been able to explain all the different observations that the the single unified dark matter model does explain. Different non-dark-matter models may be finetuned to explain some pieces of evidence, but that makes them unable to match other evidence.
Nobody is claiming that there couldn't be a nice elegant dark-matter-less model that explains everything neatly. Most astrophycisists and cosmologists would be elated if one were found, and people do keep on looking. But as long as no such thing exists, we must assume that something that walks like a duck and quacks like a duck is, indeed, a duck.
If Dark Matter exists it can be measured by it's effects on its surroundings