I'm curious about this. I saw this:


and they're suggesting that as direct searches for dark matter more and more continue to come up empty, while the gravitational observations persist, that the possibility arises that perhaps dark matter might not exist after all, and that continued hope that it exists and will be found in the next search more and more becomes a possible case of confirmation bias or special pleading, suggesting that instead some form of modification of gravity would have to be responsible (if not something else altogether).

However, what I'm curious about is the following:

  1. Is there any modified gravitational idea available that can parsimoniously account for the "tricky" observed phenomena like the Bullet Cluster and diffuse dwarf galaxies like NGC 1052-DF2 that seem to suggest the gravitational effects behave as separate matter, as well as all other suggested astronomical DM evidence, to within the margins of experimental error? As the impression I had was that modified gravity theories were getting more and more contrived,
  2. What sort of astronomical observation could distinguish between a modified theory of gravity and, say, actual dark matter that is "super dark", i.e. it ONLY interacts gravitationally (not a logical impossibility!), or at least so that any interaction it does have other that gravitation requires some ungodly high energy scale like near-Planck level to observe and thus will be forever inaccessible to direct experiments unless, perhaps, eons from now, if we or some AI or otherwise evolved descendant of us, if any exist at all, could do astronomical-scale engineering petaprojects involving petameter-scale particle accelerators capable of Planck-scale collisions? :)
  • $\begingroup$ I'm not sure that I'd worry too much about a critique of dark matter from Forbes. It is kind of like taking investment advice from Physical Review Letters. $\endgroup$
    – Jon Custer
    Aug 13 '18 at 16:39
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    $\begingroup$ Au Contraire This is a crucial question that deserves serious discussion. $\endgroup$ Aug 13 '18 at 17:40
  • $\begingroup$ @Jon Custer : It doesn't matter. The non-detection of dark matter, directly, is a thing, so far. $\endgroup$ Aug 14 '18 at 3:31
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    $\begingroup$ Sure, the non-detection is 'a thing' so far, whatever 'a thing' means in this case. But they started with the easiest to test first, and still have a fairly long list of options that they are going down. These experiments are hard to do. Anyway, experimental physicists discussing options in Physics Today would have far more credence then a blog on Forbes. $\endgroup$
    – Jon Custer
    Aug 14 '18 at 12:40
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    $\begingroup$ Forbes is the thing that people cite when they pitch random supersolid dark matter theories on Physics SE proclaiming that they're shoo-ins for the next Nobel prize. $\endgroup$
    – user191954
    Sep 6 '18 at 15:41

Well I will go and self-answer the question, following @Kyle Kanos's links in the comments to summarize the views posted and try to take a stab with my own added remarks.

The answer is no, there do not appear to be any good modified gravity alternatives. In particular, given the answer by Daniel Grumiller: Are modified theories of gravity credible? a good reason is, as I originally suggested, that the modified gravity theories can only fit a part of the data, or are too contrived. Dark matter is also not an "arbitrarily tweakable" theory with unlimited free play because the laws governing the motion of matter under a gravitational force provide the necessary constraints on its behavior so as to render it a predictive theory and thus a useful one.

There is also nothing at all logically inconsistent with the idea that dark matter could be a particle that has no interaction with ordinary matter except gravity at least before one reaches particle energies greatly surpassing those that will be amenable to technological access via particle accelerators at any time in the foreseeable future. Given the extreme success of dark matter models, if every direct-detection attempt fails then dark matter, with this caveat, will still likely be the most tenable explanation, especially if even more astronomical observations continue to be best accounted for by the laws of matter in motion. In that case, verification of its nature as such may come about indirectly through testing and confirmation of more accessible predictions of a (perhaps as-yet undiscovered) larger physical theory that would also permit for such gravity-only dark matter. Indeed, such a theory might be the most sensible to imagine in the case of a "particle physics desert" scenario which is being hinted at by the current Large Hadron Collider datasets that have shown no new elementary particles or forces than Higgs above any level of certainty that would rule out a statistical fluke. In that scenario, which is still not certain itself, it could be consistent that dark matter may not interact with ordinary matter non-gravitationally until energies reach at least around the Grand Unified scale, $10^{18}$ GeV, if not Planck scale, $10^{19}$ GeV. Such energies would require accelerators of literally astronomical dimensions (on the scales of thousands of light years in diameter) using existing magnetic confinement and acceleration technologies and thus are unlikely to be directly accessible in the near-term future if at all - that is, unless humankind becomes an interstellar civilization.


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