Well, the thing is that "dark matter" doesn't necessarily have to all be super-exotic unknown particles. It's just that the behavior we see in the universe suggests that galaxies, clusters of galaxies, clusters of clusters, etc. contain more matter than we are able to detect, since if they contained only the matter visible to us their gravitational behavior would be different.
Of course, quite a bit of it might turn out to be exotic new types of particles, but that's a question for theory and experiment.
The precedent here is quite strong, incidentally, on a few counts.
We have high confidence in our understanding of gravity on the macro scale, and that understanding has survived some crises. For example, the orbit of the planet Uranus, as originally calculated, was "wrong" -- it did not appear to be conforming to the understood behavior of gravity. One theory which was developed to explain this, and which could at the time have been criticized in the style of your comment, was that there must be another planet further out, whose gravity was perturbing Uranus' orbit. Calculating backwards from Uranus' behavior, this theory predicted the location in the sky of the additional planet... which was then promptly observed via telescope (this was the discovery of Neptune).
Similarly, we have high confidence that weakly-interacting particles -- undiscovered variants of which are a popular candidate for examples of dark matter -- exist. The neutrino is the classic example: a particle which was posited by theory as "this is the only thing that makes sense given what we see", but which would necessarily be incredibly difficult to detect on account of barely if ever interacting with anything (and thus, again, would be open to your criticism). Of course, the neutrino was eventually detected and its existence confirmed.
So there is nothing wrong or arbitrary or unusual, in terms of the history of physics, in positing something like dark matter.
>> It's just that the behavior we see in the universe suggests that galaxies, clusters of galaxies, clusters of clusters, etc. contain more matter than we are able to detect, since if they contained only the matter visible to us their gravitational behavior would be different.
I've often wondered about this(and don't know any physicists so would really love some insight on this), why do we assume that there's additional matter rather than question whether our models are correct at that scale?
>, why do we assume that there's additional matter rather than question whether our models are correct at that scale?
There are also physicists that question the models. Some scientists follow a path of inquiry based on assumptions that Einstein's theory is mostly correct -- hence a postulation of "dark matter" which leads to searching for the existence of it(1). But there are other physicists that assume there's something missing in the accepted equations that would explain the galaxy rotation speeds without any need for "dark matter".
Each competing theory tries to accumulate more and more evidence for their case until one "wins" (scientific consensus)(2). Right now, the "dark matter" line of inquiry has more scientists, more papers, and more press coverage.
(1)direct instead of indirect observation
(2)Some might say Dark Matter theory has already "won" in the marketplace of ideas
Of course, quite a bit of it might turn out to be exotic new types of particles, but that's a question for theory and experiment.
The precedent here is quite strong, incidentally, on a few counts.
We have high confidence in our understanding of gravity on the macro scale, and that understanding has survived some crises. For example, the orbit of the planet Uranus, as originally calculated, was "wrong" -- it did not appear to be conforming to the understood behavior of gravity. One theory which was developed to explain this, and which could at the time have been criticized in the style of your comment, was that there must be another planet further out, whose gravity was perturbing Uranus' orbit. Calculating backwards from Uranus' behavior, this theory predicted the location in the sky of the additional planet... which was then promptly observed via telescope (this was the discovery of Neptune).
Similarly, we have high confidence that weakly-interacting particles -- undiscovered variants of which are a popular candidate for examples of dark matter -- exist. The neutrino is the classic example: a particle which was posited by theory as "this is the only thing that makes sense given what we see", but which would necessarily be incredibly difficult to detect on account of barely if ever interacting with anything (and thus, again, would be open to your criticism). Of course, the neutrino was eventually detected and its existence confirmed.
So there is nothing wrong or arbitrary or unusual, in terms of the history of physics, in positing something like dark matter.