Dark matter questioned.

[Korgeta]

Perhaps the wrong title for the link I'm about to give but understanding dark matter IS one of my weaker points but i thought the link would be of interest, does this also affect the purpose of the LHC as it was to my understanding it was to find the 'higgs' particle.

I'll let you all comment and debate over this one.

http://www.bbc.co.uk/news/science-environment-14948730

[Napoleon the Clown]

More that the specifics of dark matter are called into question. And it's quite probable that the dominant dark matter theory is wrong. Not in the sense that saying the Earth is flat, more in the way that if even one part of a true or false question on a test is false it renders the entire question false. Newton's Laws are also considered wrong/incorrect because they don't work properly once you get to sufficiently extreme scales. They're still used, though, because the lack of precision is usually so small as to be meaningless. Think of Newton's Laws as when someone says that a certain item is three kilograms. Unless you have absurdly precise instruments you won't have a detectable change in mass if a spec of dust is introduced to the object in such a manner that it is effectively part of it. But if enough dust is introduced the change will be easily measured.

And this article nicely demonstrates what science is all about. If your test shows your hypothesis is incorrect then, unless a variable that wasn't supposed to be there was present, you've made progress.

All that being said, this is definitely an interesting development. I'd like to see more testing being done to see if there was something they missed or if the current theory is just flat-out wrong. Of course, the existence of warm dark matter does not mean that cold dark matter cannot exist.

[Steel]

Of course, the existence of warm dark matter does not mean that cold dark matter cannot exist.

Well actually one could preclude the other in the sense that you only have a finite quantity of mass that makes up dark matter and if all of that needs to be warm to have galaxies form correctly then cold dark mater doesn't get to play.

Also to consider is the conditions in the early universe, it may be if you tweak parameters like the inflaton potential or conditions around the big bang in order to get any of one type of dark matter to condense at the right point in time, then it may be impossible for any other type of dark matter to be produced earlier or later.

[Simon_Jester]

Perhaps the wrong title for the link I'm about to give but understanding dark matter IS one of my weaker points but i thought the link would be of interest, does this also affect the purpose of the LHC as it was to my understanding it was to find the 'higgs' particle.

I'll let you all comment and debate over this one.

http://www.bbc.co.uk/news/science-environment-14948730

The Higgs boson has nothing to do with dark matter, not really- it's a product of physics that will be true even if dark matter does not exist (the Standard Model of quantum mechanics). It's not a product of of general relativity- which talks about the cosmic scale of the universe and which would be in trouble if it turned out that there was no such thing as "dark matter" to provide the mass that we see pulling galaxies toward one another.

Separately and independently there's been the hope that high-energy collisions in the LHC might spontaneously create 'dark matter' particles of the types we don't see on Earth. That search is affected... but since we really have no single accepted theory about what the hell dark matter is or how much of it there is, anomalous observations don't change things much.

[HMS Conqueror]

Dark matter is just a kind of ad-hoc addition because the equations don't work otherwise, and has never been directly observed. I'd hardly equate it with Newton's Laws or the shape of the earth in the pantheon of science.

I guess the Higgs is in some way related - if the SM interpretation is correct (another big 'if'), the Higgs is what would give dark matter (if that exists...) mass, which is presumptively the only way DM interacts with the universe - but I'm not sure there's any direct overlap here. I think the real hope is that the LHC will produce and then detect actual dark matter particles - but since no one knows what, if anything, they are, that's purely speculative at this point.

[Kuroneko]

That cold dark matter had troubles with galactic formation has been known for a long time, and that warmer ones did better in simulations as well. The reason CDM is preferred at all is because it works extremely well on cosmological scales. So what is new? The quality and extent of the simulation?

Dark matter is just a kind of ad-hoc addition because the equations don't work otherwise, and has never been directly observed. I'd hardly equate it with Newton's Laws or the shape of the earth in the pantheon of science.

Bullet Cluster. MACS J0025.4-1222.

I guess the Higgs is in some way related - if the SM interpretation is correct (another big 'if'), the Higgs is what would give dark matter (if that exists...) mass, which is presumptively the only way DM interacts with the universe - but I'm not sure there's any direct overlap here.

Why dark matter has mass is completely irrelevant to its existence. The Higgs mechanism is a way to make the gauge mass have mass--the massless Goldstone bosons merge with the W and Z bosons of the electroweak force, but not the photons. The composite result is massive, which limits the range of the now-formed weak force. There's no need to postulate a new force for dark matter at all, nor a more direct connection between the Higgs field and the gravitational effects we observe as dark matter.

[HMS Conqueror]

That cold dark matter had troubles with galactic formation has been known for a long time, and that warmer ones did better in simulations as well. The reason CDM is preferred at all is because it works extremely well on cosmological scales. So what is new? The quality and extent of the simulation?

Dark matter is just a kind of ad-hoc addition because the equations don't work otherwise, and has never been directly observed. I'd hardly equate it with Newton's Laws or the shape of the earth in the pantheon of science.

Bullet Cluster. MACS J0025.4-1222.

Still quite circumstantial. I'm not saying that the DM hypothesis is worthless or wrong, just that it doesn't warrant being put on the same pedestal as firmly established theories like that the world isn't flat, or even Newton's laws which are firmly established within limits.

I guess the Higgs is in some way related - if the SM interpretation is correct (another big 'if'), the Higgs is what would give dark matter (if that exists...) mass, which is presumptively the only way DM interacts with the universe - but I'm not sure there's any direct overlap here.

Why dark matter has mass is completely irrelevant to its existence. The Higgs mechanism is a way to make the gauge mass have mass--the massless Goldstone bosons merge with the W and Z bosons of the electroweak force, but not the photons. The composite result is massive, which limits the range of the now-formed weak force. There's no need to postulate a new force for dark matter at all, nor a more direct connection between the Higgs field and the gravitational effects we observe as dark matter.

I didn't say anything about a new force(?), just that the only connection I could think of between the Higgs and DM is that DM is observed (possibly) only through the gravitational interaction, while the Higgs is thought to be responsible for the mass of elementary particles.

[Kuroneko]

Bullet Cluster. MACS J0025.4-1222.

Still quite circumstantial. I'm not saying that the DM hypothesis is worthless or wrong, just that it doesn't warrant being put on the same pedestal as firmly established theories like that the world isn't flat, or even Newton's laws which are firmly established within limits.

The above shows that it is directly observed, so why call it an ad hoc hypothesis? I'm not sure how any astronomical observation could be any less "circumstantial". Is the mass of Jupiter circumstantial?

I didn't say anything about a new force(?), just that the only connection I could think of between the Higgs and DM is that DM is observed (possibly) only through the gravitational interaction, while the Higgs is thought to be responsible for the mass of elementary particles.

Ah, I thought you were saying there was a necessary connection, so I aimed to cover both kinds of interactions: giving masses to force carriers and more direct interactions.

[Ziggy Stardust]

The above shows that it is directly observed, so why call it an ad hoc hypothesis? I'm not sure how any astronomical observation could be any less "circumstantial". Is the mass of Jupiter circumstantial?

I'm not disagreeing with you or anything, but dark matter wasn't "directly observed" in either of those instances. Or do you just not understand what "directly observed" means? That said, those two instances do provide great evidence in support of the theory, and is as concrete as a lot of other astronomical observation. The truth is most of those measurements can only be taken indirectly. It doesn't make the evidence any stronger or weaker, really, but saying it was "directly observed" is a stretch by any means.

[Bakustra]

The above shows that it is directly observed, so why call it an ad hoc hypothesis? I'm not sure how any astronomical observation could be any less "circumstantial". Is the mass of Jupiter circumstantial?

I'm not disagreeing with you or anything, but dark matter wasn't "directly observed" in either of those instances. Or do you just not understand what "directly observed" means? That said, those two instances do provide great evidence in support of the theory, and is as concrete as a lot of other astronomical observation. The truth is most of those measurements can only be taken indirectly. It doesn't make the evidence any stronger or weaker, really, but saying it was "directly observed" is a stretch by any means.

Uh, yes it was. The centers of mass of those two structures are offset from the centers of the visible mass. Dark matter is defined by its near-invisibility except via gravity, not by being a specific type of non-baryonic massive particle. That's like saying we haven't directly observed the black hole at the center of the Milky Way, because we haven't produced a picture of the even horizon. We've directly observed brown dwarfs, red dwarfs, and neutron stars too, all of which can potentially serve as MACHOs, although those cannot be a significant percentage of dark matter.

[HMS Conqueror]

I wouldn't put black holes (at least, actual singularities) in the same category as really firmly experimentally supported theories (like the non-flatness of earth, for which there is an enormous amount of direct observational evidence) either. A lot of astrophysics is in fact slightly dubious. Not necessarily wrong, but constrained by our physical limitations to observe, forcing us to draw conclusions from somewhat sketchy evidence. Looking at gravitational deflection, of light or of matter, isn't a direct observation of dark matter, it is an indirect effect from which the presence of dark matter is inferred. As far as I'm aware, the jury is still out on what dark matter even is - give that as you say it seems it can't be explained by 'normal' baryons.

[Kuroneko]

The above shows that it is directly observed, so why call it an ad hoc hypothesis? I'm not sure how any astronomical observation could be any less "circumstantial". Is the mass of Jupiter circumstantial?

I'm not disagreeing with you or anything, but dark matter wasn't "directly observed" in either of those instances. Or do you just not understand what "directly observed" means?

Direct observation means you look at something and observe its properties. This is about as direct as it gets, unless for some reason you want to limit "direct" to only things like the pointer readings on an instrument, in which case no one can even directly observe the voltage of a battery.
Maybe you're confusing natural and controlled experiments, which would be a different issue altogether.

I wouldn't put black holes (at least, actual singularities) in the same category as really firmly experimentally supported theories (like the non-flatness of earth, for which there is an enormous amount of direct observational evidence) either. A lot of astrophysics is in fact slightly dubious. Not necessarily wrong, but constrained by our physical limitations to observe, forcing us to draw conclusions from somewhat sketchy evidence.

We have nearly the same reasons to believe that Jupiter is much more massive than Earth as to believe that the mass of those objects is much higher than the the amount of visible matter in them.

While I'm glad you've progressed from "ad hoc" to "slightly dubious", the existence of dark matter is still a lot less dubious than you think. I don't know why the comparison to non-flatness of the Earth is even relevant. If that's an argument, one might as well say that the Earth being non-flat is ad hoc and dubious because 2+2=4.

[HMS Conqueror]

Not everything that is slightly dubious is ad hoc. DM is an ad hoc addition - no one knows what it is or thought it would be there before they realised observations couldn't be explained otherwise -, but black holes are a natural consequence of GR and were predicted long before they were (probably) discovered.

Comparison to non-flatness and Newton's Laws was made by someone earlier in the thread. And I don't understand your desire to reduce everything to boolean right or wrong. It's more like probabilities. 2+2=4 is close enough 100%. Non-flatness of the world is 99.9999%. Dark matter is, what? I'm not sure I'd bet on it even at 50-50. Maybe you would, but would you bet on it at 999999-1?

[Kuroneko]

And I don't understand your desire to reduce everything to boolean right or wrong. It's more like probabilities. 2+2=4 is close enough 100%.

Please don't strawman. The argument above explicitly uses relative certainty, and that was the entire point of comparison. So I've no idea where you're getting that from.

Either you don't know what "ad hoc" means or you're unaware of what's going on.

When it was observed that Cygnus X-1 was a compact anomalous X-ray source, they made detailed models of all the types of processes they could think of that it could be. Eventually, empirical evidence strongly favored 'blach ole'. It doesn't mean scientists have thought of everything or 'must' be right in some other way. What's important is that a lot of investigations produced a lot of reasonable alternatives, and they have been eliminated by observation. Note that it doesn't matter if anyone actually expected X-ray sources of the type to be around. What matters is that the possibilities are genuinely investigated and alternatives eliminated. (If we're interested in black holes in general, there's broader and better evidence than just Cyg X-1, but that's another issue.)

DM is an ad hoc addition - no one knows what it is or thought it would be there before they realised observations couldn't be explained otherwise -, but black holes are a natural consequence of GR and were predicted long before they were (probably) discovered ... Comparison to non-flatness and Newton's Laws was made by someone earlier in the thread.

If we count personal expectations to gauge how surprising dark matter is, the idea that there may be some 'corpuscules' with total mass several orders of magnitude higher than stars and nebulae has been around for over a century.

But that's not important. What is so is that the galactic rotation curve measures the enclosed mass directly by Newton's laws. (Which you've admitted have high certainty, so there's a bit of inconsistency there in calling it as not directly observed.) So either

• There's mass there.
• The gravity we know doesn't apply at those scales.

Both of these categories have many proposals within them. Objects like the Bullet Cluster is that they actually rule out the various proposals in the second category.

You're misconstruing the 'dark matter problem'. It's not that we've no idea what it is. We have a myriad of ideas of what it is. Some of it based on physics we see in the lab, some on natural extensions of motivated by issues other than dark matter, and some based on natural extensions created before dark matter was even discovered. The problem is the mostly the opposite: there are far too many good ideas, and not enough empirical data to rule out enough of them.

Dark matter is, what? I'm not sure I'd bet on it even at 50-50. Maybe you would, but would you bet on it at 999999-1?

I would, yes. And at more skewed odds than that besides. We know, with high certainty, that there is mass and that it is dark. It's nature is highly uncertain, but that's not what you're arguing at all.

[HMS Conqueror]

And I don't understand your desire to reduce everything to boolean right or wrong. It's more like probabilities. 2+2=4 is close enough 100%.

Please don't strawman. The argument above explicitly uses relative certainty, and that was the entire point of comparison. So I've no idea where you're getting that from.

Either you don't know what "ad hoc" means or you're unaware of what's going on.

When it was observed that Cygnus X-1 was a compact anomalous X-ray source, they made detailed models of all the types of processes they could think of that it could be. Eventually, empirical evidence strongly favored 'blach ole'. It doesn't mean scientists have thought of everything or 'must' be right in some other way. What's important is that a lot of investigations produced a lot of reasonable alternatives, and they have been eliminated by observation. Note that it doesn't matter if anyone actually expected X-ray sources of the type to be around. What matters is that the possibilities are genuinely investigated and alternatives eliminated. (If we're interested in black holes in general, there's broader and better evidence than just Cyg X-1, but that's another issue.)

I completely agree - DM is a not unreasonable hypothesis, and it has some empirical backing, but it is not as firmly established as many older and (in a pop-sci sense) duller theories with which it is compared.

DM is an ad hoc addition - no one knows what it is or thought it would be there before they realised observations couldn't be explained otherwise -, but black holes are a natural consequence of GR and were predicted long before they were (probably) discovered ... Comparison to non-flatness and Newton's Laws was made by someone earlier in the thread.

If we count personal expectations to gauge how surprising dark matter is, the idea that there may be some 'corpuscules' with total mass several orders of magnitude higher than stars and nebulae has been around for over a century.

That's extremely disingenuous. There was no widely-accepted theory which predicted dark matter as a natural consequence before it was "discovered", while black holes are a natural consequence of the Schwarzschild solution of the Einstein field equations in GR, which was worked out decades before anyone had seen good evidence that they actually existed. Previous prediction is irrelevant to whether the things are real, I agree - but not to whether the hypothesis is ad hoc. If we had only Newtonian gravity and black holes were discovered, they would have to be made an ad-hoc addition to a theory with which they're not obviously compatible - even though we'd be forced to admit from the observations they exist regardless. As it was, they slotted in readily.

But that's not important. What is so is that the galactic rotation curve measures the enclosed mass directly by Newton's laws. (Which you've admitted have high certainty, so there's a bit of inconsistency there in calling it as not directly observed.) So either

• There's mass there.
• The gravity we know doesn't apply at those scales.

Both of these categories have many proposals within them. Objects like the Bullet Cluster is that they actually rule out the various proposals in the second category.

You're misconstruing the 'dark matter problem'. It's not that we've no idea what it is. We have a myriad of ideas of what it is. Some of it based on physics we see in the lab, some on natural extensions of motivated by issues other than dark matter, and some based on natural extensions created before dark matter was even discovered. The problem is the mostly the opposite: there are far too many good ideas, and not enough empirical data to rule out enough of them.

Newton's laws are firmly established within limits - essentially I wouldn't say that the probability of the second class of hypotheses being correct has reduced to less than 0.00001%. And having dozens of ideas, none of which are confirmed or falsified, is the same as not knowing.

Dark matter is, what? I'm not sure I'd bet on it even at 50-50. Maybe you would, but would you bet on it at 999999-1?

I would, yes. And at more skewed odds than that besides. We know, with high certainty, that there is mass and that it is dark. It's nature is highly uncertain, but that's not what you're arguing at all.

Fair enough. How about it? £1 vs £1,000,000. (although, defining who wins and when will probably be a nightmare)

[Kuroneko]

I completely agree - DM is a not unreasonable hypothesis, and it has some empirical backing, but it is not as firmly established as many older and (in a pop-sci sense) duller theories with which it is compared.

Well, for one thing, DM is not a theory, but an observation.

That's extremely disingenuous.

That's why I called it "personal expectations" and followed up with "but that doesn't matter," followed by an explanation of why neither that expectation nor your demand for hard theoretical prediction is relevant.

And dark matter was a natural expectation of established physics years prior to its discovery. Why do you think scientists immediately jumped on hot dark matter models? Because that corresponds to the most common light, noninteractive particle they knew, the neutrino (since then, likely more massive varieties have been discovered).

Previous prediction is irrelevant to whether the things are real, I agree - but not to whether the hypothesis is ad hoc.

That's true at least for the case where further developments don't confirm it. But in the interests of preventing confusion, could you state explicitly what theory you're talking about in this context? (You called DM a theory earlier, but that doesn't make sense here.) I gather it's Newton or GTR.

Newton's laws are firmly established within limits - essentially I wouldn't say that the probability of the second class of hypotheses being correct has reduced to less than 0.00001%. And having dozens of ideas, none of which are confirmed or falsified, is the same as not knowing.

Then unless you rhetorically understated your confidence in such, I think we're making progress.
Weren't you arguing for split on the order of 50/50 earlier? Do you still believe that it's about even odds that Newton/GTR is very massively wrong on galactic scales?

Fair enough. How about it? £1 vs £1,000,000. (although, defining who wins and when will probably be a nightmare)

Whatever happens, explicatating the claims made is a good start.

[HMS Conqueror]

I completely agree - DM is a not unreasonable hypothesis, and it has some empirical backing, but it is not as firmly established as many older and (in a pop-sci sense) duller theories with which it is compared.

Well, for one thing, DM is not a theory, but an observation.

It's a hypothesis (and not the only one, though the most popular and arguably the best supported) that seeks to explain certain direct observations, in particular divergence of galaxy rotation curves from the Newtonian prediction. I don't think the way I described it is inaccurate at all.

That's extremely disingenuous.

That's why I called it "personal expectations" and followed up with "but that doesn't matter," followed by an explanation of why neither that expectation nor your demand for hard theoretical prediction is relevant.

And dark matter was a natural expectation of established physics years prior to its discovery. Why do you think scientists immediately jumped on hot dark matter models? Because that corresponds to the most common light, noninteractive particle they knew, the neutrino (since then, likely more massive varieties have been discovered).

I'm not clear what you mean here. In a sense, dark matter (as in, regular matter of types already known to exist) was expected - no one expected every part of the universe to be radiating. But the contribution from that sort of dark matter is far too small to explain the gap in what we can see and what the inferences from gravitational motion seem to be telling us. That's why all the journalists and popsci authors care about dark matter, rather than it being totally mundane.

Previous prediction is irrelevant to whether the things are real, I agree - but not to whether the hypothesis is ad hoc.

That's true at least for the case where further developments don't confirm it. But in the interests of preventing confusion, could you state explicitly what theory you're talking about in this context? (You called DM a theory earlier, but that doesn't make sense here.) I gather it's Newton or GTR.

Black holes were anticipated by GR. (I think that's what you're asking?)

Newton's laws are firmly established within limits - essentially I wouldn't say that the probability of the second class of hypotheses being correct has reduced to less than 0.00001%. And having dozens of ideas, none of which are confirmed or falsified, is the same as not knowing.

Then unless you rhetorically understated your confidence in such, I think we're making progress.
Weren't you arguing for split on the order of 50/50 earlier? Do you still believe that it's about even odds that Newton/GTR is very massively wrong on galactic scales?

I said I wasn't sure at 50-50 - that wasn't rhetoric, I'm actually not. I would, however, take a bet against dark matter far more readily than one against the non-flatness of the earth or Newton's laws (I doubt I'd accept such a bet on any terms, unless a totally irrelevant sum for a joke), which has been my point all along.

Fair enough. How about it? £1 vs £1,000,000. (although, defining who wins and when will probably be a nightmare)

Whatever happens, explicatating the claims made is a good start.

It's extremely difficult to frame fairly and concisely - perhaps something like that no one will discover a WIMP in the next 20 years? (leaving aside difficulty of anyone actually collecting the money after that time).

[Kuroneko]

It's a hypothesis (and not the only one, though the most popular and arguably the best supported) that seeks to explain certain direct observations, in particular divergence of galaxy rotation curves from the Newtonian prediction. I don't think the way I described it is inaccurate at all.

I think it is, and I've asked you to back that up...
First, your information is wrong, because the dark matter responsible for galactic rotation curves has been anticipated by about four decades prior based on studies of clusters.
Second...

Black holes were anticipated by GR. (I think that's what you're asking?)

No, it's not what I'm asking at all. You said dark matter is ad hoc. For that statement to make sense, there must be a theory that's in danger of being falsified that DM 'rescues'. I am simply asking to specify what theory you're talking about. In a manner at least a bit more precise than the oblique, roundabout references you've been making.

I've guessed that you meant Newtonian gravity, and you seem to implicitly confirm that above. But I don't see how Newtonian gravity makes a prediction about galaxy rotation curves. Perhaps you have some basic theory about galactic properties that incorporates Newtonian gravity in mind?

I'm not clear what you mean here. In a sense, dark matter (as in, regular matter of types already known to exist) was expected - no one expected every part of the universe to be radiating. But the contribution from that sort of dark matter is far too small to explain the gap in what we can see and what the inferences from gravitational motion seem to be telling us.

How do you know it's too small? Is that statement based on the same theory that you're avoiding to specify?

I'm asking you to do this because it is directly relevant to what degree something is ad hoc. If you don't, then it's just like many other once-newly discovered facts about the universe.

I said I wasn't sure at 50-50 - that wasn't rhetoric, I'm actually not. I would, however, take a bet against dark matter far more readily than one against the non-flatness of the earth or Newton's laws (I doubt I'd accept such a bet on any terms, unless a totally irrelevant sum for a joke), which has been my point all along.

And my point your position is self-contradictory, because a bet against dark matter is necessarily also a bet against Newton's laws. Hence the two categories I mentioned above.

It's extremely difficult to frame fairly and concisely - perhaps something like that no one will discover a WIMP in the next 20 years? (leaving aside difficulty of anyone actually collecting the money after that time).

That criterion misses the point of the disagreement, which is basically regarding whether it's possible to confirm the existence of mass with high certainty without any significant specification about its microscopic structure.

[HMS Conqueror]

I don't see where you're going with this - I started drafting a reply but everything you have written seems to be just result of either misreading or misunderstanding what I am saying. So I'll just restate my position, hopefully with greater clarity, and you can tell me which part you disagree with.

Overall claim: The standard interpretation of dark matter is substantially less certain than the non-flatness of the earth or Newtonian gravity within its accepted limits.

Justification: The dark matter intepretation of astronomical observations requires the existence of particles that have not been (and may not ever be) observed (WIMPs, etc.). The possibility that observations are explained instead by modified dynamics, or something else we haven't thought of (an "unknown unknown") should therefore be taken seriously. This is in contrast to the non-flatness of the earth, or Newtonian gravity, which have been directly verified in all significant particulars repeatedly.

[Kuroneko]

I don't see where you're going with this - I started drafting a reply but everything you have written seems to be just result of either misreading or misunderstanding what I am saying. So I'll just restate my position, hopefully with greater clarity, and you can tell me which part you disagree with.

If all you have stated was a relative comparison to something like the non-flatness of Earth, I wouldn't have bothered commenting on your post in the first place. But you've claimed something much stronger than that several times.

In particular, you stated that dark matter is an ad hoc fit to galaxy rotation curves. This is wrong on many levels, not only historically because the first evidence for dark matter predates measurements of galactic rotation curves by four decades, but also because it shows a complete lack of understanding of "ad hoc" means. Some of the types of evidence for dark matter and things that cross-check it: cosmic microwave background, galactic clusters and superclusters, galactic rotations, lensing, baryon acoustic oscillations, Lyman-alpha lines, etc.

If all dark matter had going for it was that it "explained" the galactic rotation curves, well, you'd have somewhat of a stronger case. But that's not even remotely close to the actual state of affairs. If you look at a galaxy and measure its dark matter distribution from its stellar rotation cuves, that measurement has to be consistent with the behavior of the cluster the galaxy is in, and a galactic should be consistent with the measurement from CMB, and so on and so forth. Those checks are across many types of observations and across a truly staggering scale: from the sub-galactic to the entire observable universe.

I didn't put the formation of galaxies in the above list because that's what this thread is about. But even then, it doesn't look to be anything more than a correction of the relative distribution of cold vs warm dark matter rather than any evidence against dark matter. Whatever the relative certainty to Earth's non-flatness is, it should be very clear by now that when you make an observation and cross-check it with many others over many types of phenomena and an extremely very wide scale, you're about as far away from "ad hoc" as you can possibly be.

Justification: The dark matter intepretation of astronomical observations requires the existence of particles that have not been (and may not ever be) observed (WIMPs, etc.). The possibility that observations are explained instead by modified dynamics, or something else we haven't thought of (an "unknown unknown") should therefore be taken seriously. This is in contrast to the non-flatness of the earth, or Newtonian gravity, which have been directly verified in all significant particulars repeatedly.

As far as "unknown unknowns" go, it's completely like the non-flatness of Earth or anything else whatsoever, because one can never be sure whether one has really thought of all the possibilities.

Modified dynamics have lots of trouble because of the vast observational range for dark matter. If you have the rotation curves down pat, what about clusters? That's was the whole point of the Bullet Cluster reference: it strongly ruled out such things. (Actually MOND's relativistic version had lot more trouble than that--it did not get a handle on individual stars without introducing massive particles.) An example of a family of modified gravity theories that actually has a chance, f(R), is basically equivalent to standard Λ-less GTR plus a massive scalar field.

[HMS Conqueror]

I don't see where you're going with this - I started drafting a reply but everything you have written seems to be just result of either misreading or misunderstanding what I am saying. So I'll just restate my position, hopefully with greater clarity, and you can tell me which part you disagree with.

If all you have stated was a relative comparison to something like the non-flatness of Earth, I wouldn't have bothered commenting on your post in the first place. But you've claimed something much stronger than that several times.

In particular, you stated that dark matter is an ad hoc fit to galaxy rotation curves. This is wrong on many levels, not only historically because the first evidence for dark matter predates measurements of galactic rotation curves by four decades, but also because it shows a complete lack of understanding of "ad hoc" means. Some of the types of evidence for dark matter and things that cross-check it: cosmic microwave background, galactic clusters and superclusters, galactic rotations, lensing, baryon acoustic oscillations, Lyman-alpha lines, etc.

It is ad-hoc in the sense that it is an assumed explanation for certain unanticipated developments but that has not been independently observed. If this doesn't agree with the sense in which you would use the term, we will have to agree to disagree.

Justification: The dark matter intepretation of astronomical observations requires the existence of particles that have not been (and may not ever be) observed (WIMPs, etc.). The possibility that observations are explained instead by modified dynamics, or something else we haven't thought of (an "unknown unknown") should therefore be taken seriously. This is in contrast to the non-flatness of the earth, or Newtonian gravity, which have been directly verified in all significant particulars repeatedly.

As far as "unknown unknowns" go, it's completely like the non-flatness of Earth or anything else whatsoever, because one can never be sure whether one has really thought of all the possibilities.

Not really, since an 'unknown unknown' still has to be consistent with the previous evidence. So, say, it's possible mankind was experiencing collective delusion and actually the world isn't flat. In a philosophical sense, ok, that could happen. But for DM, an 'unknown unknown' could explain the indirect inferences without involving 'dark matter' at all. That is the difference I've been trying to elucidate.