The word “stuff” there is exactly the point. It’s a lot more specific than “just a question mark” -- the hypothesis is that it’s some exotic form of matter.
If something like MOND gains popularity and there’s no longer a need to postulate invisible matter, it would be fair to say that dark matter is incorrect and outdated, just like ether or phlogiston.
That would be fair. However, from what I've read MOND is relatively unlikely. If its proponents can address the biggest difficulties (e.g. not every galaxy appears to have "dark matter") and make it somewhat more feasible, then I'd agree the name is insufficiently broad.
> MOND and the like falls into the category of "dark matter theory"
No, it doesn't. See my response to your other post upthread about the two hypotheses. "Dark matter" is one hypothesis; "MOND" and friends are the other.
No, it is unfair to call MOND (and at least much of "the like") as a dark matter theory, because it manifestly generates a solution (or solutions) in the absence of any matter at all.
MOND is characterized by an interpolating function between its fundamental constant a_0 and acceleration, such that in the "deep-MOND" regime, F = m \frac{a^2}{a_0} rather than Newton's F = ma.
Both MOND and Newton work in everywhere-vacuum, and in everywhere-vacuum perturbed by a point mass. Neither the vacuum case nor the point mass case qualifies as a "dark matter theory" any more than the vacuum Schwarzschild solution of General Relativity does. These are model universes where one can calculate a Gauss law for gravity.
MOND would behave differently for deep deep extragalactic space with small icy body moving around a large gassy body than Newton would, or than we would get from e.g. a roughly Einstein-de Sitter solution.
Milgrom has pursued several relativistic corrections to the simple MOND formula, and they generally allow for solutions where the stress-energy tensor is everywhere 0, and everywhere 0 except for one point. Milgrom originated MOND. He has collected numerous thoughts and many links at http://www.scholarpedia.org/article/The_MOND_paradigm_of_mod...
When applied to large systems like galaxies and especially galaxy clusters, Milgrom's corrections may be insufficient, and other corrections (catalogued well in https://arxiv.org/abs/1112.3960 section 7). Several of these are very hard to make work with the stress-energy tensor everywhere zero, and those (certainly not all of them) one might ungraciously call "just another dark matter theory". On the other hand, several of these theories admit plausible gravitational radiation in the total absence of matter, just as does General Relativity. (Gravitational radiation is not a feature of Newton or the simple original Milgrom formula.) McGaugh runs tritonstation and is a well known sceptic of particle dark matter.
Relativistic dark matter theories deliberately introduce nonzeros into the stress-energy tensor. You need a distribution of dark matter and trace out how that generates curvature, or you don't have a (relativistic) dark matter theory. You can also of course set out a distribution of cold dark matter and see how it behaves under F = ma or F = m \frac{a^2}{a_0}. Neither MOND nor Newton requires dark matter; likewise neither MOND nor Newton is incompatible with dark matter. Likewise, General Relativity doesn't require dark matter. But one gets more physically realistic solutions at a wide range of scales when one adds dark matter to a defined large-scale distribution of matter and radiation. MOND or the various attempts at generally covariant Milgrom gravitation would also behave differently when one adds dark matter to a large-scale distribution of matter and radiation. That is unlikely to produce more physically realistic solutions to a wide range of scales, however.
Indeed, Milgrom et al's most concrete argument is that their formula for gravitation is worse if one adds in a distribution of dark matter, and that their formula is better than e.g. a Lemaître-Tolman-Bondi solution equipped with a distribution of cold dark matter. That's as much a non-dark-matter theory as I think it is possible to have!
Forgive me, but I was under the impression the phrase "dark matter theory" can confusingly refer to something that generally explains galactic rotation curves whether or not it's a theory that involves dark matter. What would you call the general category of theories which explain galactic rotation curve deviations?
The redshifting things are mostly molecular gas clouds which we would expect would orbit in a Keplerian fashion given the overall distribution of luminous matter and occluding/extinguishing dust visible in galaxies. Astrophysicists love hydrogen gas clouds because they ionize easily from background emitters (stars, quasars) producing very sharp spectral lines, and you can pick up their changes by looking on different sides of an edge-on spiral or are at nearly perpendicular angles to the rotational axis of spheroidals. For giant ellipticals, which have practically zero axial rotation, an instrument that gets a sweeping view of them will see a more chaotic red-and-blue shifting of the Lyman-alpha hydrogen line, rather than an obvious gradient between advancing and retreating edges. (Face-on spirals often also have gas clouds on excursions back and forth in front of the disk plane, where "forth" means "towards us" means "relatively blueshifted" and that's hard to explain without a halo rather than a ring of dark matter).
So really it's a question explaining the non-Keplerian https://en.wikipedia.org/wiki/Peculiar_velocity of structures from ionized hydrogen to molecular clouds to supernovae with highly predictable light curves.
While basic MOND can explain edge-on spirals and axis-perpendicular spheroidals and lenticulars, it does a bad job with face-on spirals, ellipticals, and lots of other unusual shapes in the galaxy zoo. It also doesn't do well with entire clusters, where there are enormous gas clouds between the member galaxies, and often a bright quasar (or at least central galaxy) really lighting them up. Corrections to basic MOND patch some of these up, mostly starting by adding extra terms to the initial basic Milgrom formula which modified F = ma.
A ball-like distribution of dark matter around these very large structures (galaxies, clusters) on the other hand is a decent explanation, with the major drawback that the only sufficiently transparent stuff we've produced in labs is simply too light to stay constrained within these structures (electron neutrinos and their antis would fly away to infinity, for example, rather than stick around in a hierarchy of halo/shell/ball structures; free neutrons aren't stable). We also don't have a really good explanation about why these proposed dark matter structures don't have a strong density gradient leading to grossly non-Keplerian orbits of stars in the cores of galaxies (including our own). That's why you get wildly speculative ideas like phase transitions, "dark chemistry", dark-matter/dark-matter annihilation, and so on. There are similar questions about why the outer reaches of dark matter halos don't blow off into infinity rather than lingering around. ( See the excellent III A and III E of Adams & Laughlin https://arxiv.org/abs/astro-ph/9701131 or the Binney & Tremaine textbook https://press.princeton.edu/books/paperback/9780691130279/ga... )
Good explanations should cover the entire lifetime of a galaxy including the things that lead up to it and its ultimate decay. This also means it should be good for the oldest galaxies that we can see. It also means that as we get better at simulating initial value problems we can take a large number of observed quantities of a highly-redshifted galaxy and evolve that initial snapshot step by step such that after trillions of steps we end up with something that looks remarkably like a galaxy with low redshift. We're not really there yet. Work harder, supercomputer builders! :D :D
The problem, IIRC, is that we don't have an explanation of why those distributions of DM are balls around galaxies and not balls in the intergalactic medium. It seems like the distribution of lambda-CDM could be anything. So much so, that people are postulating that the core of the milky way galaxy could be an ultradense ball of lambda-CDM.
Matter fell into place wherever it could, forming overdense areas. More matter tended to fall into overdense areas from underdense areas, and this allowed the underdense areas to expand more easily while the overdense areas were constrained to expand more slowly or even to collapse. Dark matter, not being able to collide and radiate away the collision-energy collapses much more slowly than heavy matter.
Dark matter connects fluctuations in the cosmic microwave background with large scale structures -- the cosmic web -- extremely well, as long as it moves nonrelativistically (that's why it's "cold" compared to electron neutrinos which move ultrarelativistically) and collapses very slowly.
> ultradense ball of lambda-CDM
Mu?
Lambda (capital greek letter) is the term that encodes the expansion of space and the consequent separation of clusters of galaxies from one another. It's irrelevant within clusters of galaxies, let alone individual galaxies. The solar system is not expanding at all, and neither is Manhattan.
Lambda is also a constant. It doesn't form balls, there are no overdensities or underdensities, it has the same tiny value at every point in spacetime.
CDM is cold dark matter, and your phys.org link discusses evidence for a possible explanation about why there isn't a high density "ball" of dark matter in the galactic centre compared to around our solar system: maybe dark matter can interact electromagnetically under conditions found in galactic cores. The electromagnetic interaction in this study would produce a gamma ray and the vanishing of a dark matter particle. This could be some complicated way dark matter can self-annihilate in the presence of very hot standard model gas like that around black holes or produced by supernovae, for instance. Whatever the specifics, this conversion of dark matter to gammas would keep the central dark matter relatively sparse.
(Other mechanisms for avoiding high dark matter densities in galaxy cores are available. These do have a good collective name: galactic outflow, of which dark matter heating and dark matter decay are subcategories that are at the root of your phys.org link. Generally dark matter turns into hot matter, or drags hot matter along with it for a ride. Outflow must enrich the metallicity of stars far from galactic cores, and quench core star formation, and these generate observables for individual proposed mechanisms, and astronomers are already happily gathering up data that probe those observables. Good proposals for outflow mechanisms also work on the scale of structures like the Coma Cluster).
