Dr Becky explaining the MOND theory and why it's invalid --> https://www.youtube.com/watch?v=HlNSvrYygRc
She knows her stuff and approaches things rationally.
and Wikipedia editors seem to agree, with further references https://en.m.wikipedia.org/wiki/Becky_Smethurst
The video cites two papers, one for MOND and one against, so you can look up the papers.
It's better than the OP article, which ignores the counterarguments against MOND.
But more important than that, if you want more, you can read the paper her entire video is based on [1]
Also be aware that Sabine Hossenfelder, also a popular science YouTuber and published physicist released a video supportive of that paper, even though she was somewhat in favor of MOND before [2]. She even co-authored a paper about it [3] which she presented in a video [4]
[1] https://doi.org/10.1093/mnras/stad3393
[2] https://www.youtube.com/watch?v=i4lu9AxRtqA
While it matches well many experimental facts, there also other experimental facts that appear to contradict it.
There is an essential difference between a MOND-like theory and a dark matter based theory.
For a MOND-like theory one has to choose some mathematical relationship that determines the gravitational forces, given the observed distribution of matter in the Universe. Then one must compute the expected movements and compare them with the observed movements, to verify or falsify the postulated mathematical model.
On the other hand, any theory based on dark matter does not have any predictive power or any usefulness. Because the observed movements of the bodies in the Universe cannot be explained by the conventional mathematical model, one adds arbitrarily dark matter wherever it is necessary to remove the discrepancies in the observed movements.
When one is free to add dark matter, then all mathematical models for gravitation become equivalent and none can be used to predict what we observe.
Unless an alternative method for observing dark matter would be discovered, using it is just an euphemism for avoiding to recognize that the current model of gravitation is not accurate enough.
We see this all the time when MOND is tested. It ends up failing, but then the proponents say "oh, well, it's not that MOND we're talking about". It's like some sort of pseudoscientific cockroach that keeps escaping after you crush it with a shoe.
Any theory based on dark matter is not falsifiable, regardless what model is used for gravitation, because for now there is no constraint on the distribution of the dark matter.
The only way for the theory of dark matter to become a scientific theory is to discover an alternative way to determine where the dark matter is located, besides placing it wherever necessary to remove the discrepancies between the observations of the movements of the celestial bodies and the predictions of the current model of gravitation.
I always got the impression that when Dark Matter was initially labelled as such, it was just a name for the discrepancy between theoretical models and observations; and that the name itself seems to have driven this idea that it's the observations that are wrong and not the models.
Personally, when discussing Dark Matter vs. MOND, I think neither should be treated as a concrete "theory", but simply a different perspective on where the problem lies. "Dark Matter" is the idea that our observations are incomplete, and MOND is the idea that our theoretical models are wrong.
Hopefully this conundrum is resolved within my lifetime, because I'd love to know what the answer is. It would be absolutely wild if they're both right i.e. that our observations are incomplete and our models are wrong.
I'd say that's a given regardless of the DM mystery.
It's consensus that QM and GR are incompatible and that we need a new theory out of which both of these come out as a special case. String theory was considered a hopeful contender for that for a while.
And that we haven't observed everything to a satisfying degree yet should be obvious.
> Isn't this essentially the same problem with Dark Matter though? They keep looking for it, not finding it and proclaiming "well, it must be somewhere else!".
No (unless you mean "where" in parameter space), we have a pretty good idea where it is thanks to gravitational lensing surveys. We don't what it is.
But when we look at galaxies and so on, things don't seem to add up if you only look at the visible matter.
So, there could be a lot of dark matter we can't see. Or GR might not be correct and MOND (or a relativistic formulation like AQUAL) might be the right answer.
It's true I guess that if GR is correct, then we need some kind of dark matter to explain observations. But it might not be correct.
Adding GR to the picture doesn't take away the need for dark matter to account for observations.
Whenever the facts predicted by a theory do not match the experimental facts, one could say that the theory has predicted the existence of an unknown factor that has affected the experiments.
Nevertheless, until there is an alternative way to determine the existence of that unknown factor, the right way is to simply say that there is a mismatch between predictions and observations and the reason for this mismatch must be determined in the future. For now, the current theory is not accurate enough.
For instance, when some planetary movements did not coincide with the predictions, it was supposed that perhaps there exists an extra planet which explains the discrepancies between predictions and observations.
This supposition was confirmed only when Neptune was also observed with a telescope. If Neptune had never been observed, perhaps it would have been discovered that the mathematical model of gravitation must be improved.
For now, there are discrepancies between observations and the predictions of the current mathematical model of gravitation. Like in the cases of Neptune and Pluto, there is a supposition that perhaps there exists some kind of dark matter that would be the cause of the discrepancies.
Until the moment when an alternative way to determine the existence of dark matter will be discovered, like the optical observations of Neptune and Pluto, the existence of dark matter remains just a hypothesis that cannot be used for any practical purpose, because it cannot predict anything. Dark matter can be added arbitrarily in any place and this can make any theory of gravity match the observations.
Therefore now we have galaxies that are supposed to be rich in dark matter and galaxies that are supposed to be poor in dark matter, in order to fit the observations, but without any a priori rule that could be used to predict this.