| HN Mirror

https://tritonstation.com/2016/12/23/crater-2-the-bullet-clu...

Far from settled: https://arxiv.org/abs/1003.0939

But armchair physicists love to shriek 'bullet cluster' every time because they aren't aware of the extent of their ignorance. Considering your HN bio I'd comfortably classify you as "knows just enough to be dangerous". Fair, given your generalization knowing nothing about me?

More reading for your benefit, from a working astrophysics professor specializing in ultra-diffuse galaxies:

https://tritonstation.com/2017/03/06/lcdm-has-met-the-enemy-...

https://tritonstation.com/2016/07/30/missing-baryons/

JumpCrisscross 987 days ago

Lots of ground between settled science and “no good reason is ever given.”

There are strong arguments for dark matter (broadly, not just Lambda CDM). They are inconclusive. The bullet cluster is less strong evidence for dark matter than good evidence against common armchair refutations. (I agree with your author that it doesn’t rule out MOND as is commonly claimed.)

The arguments inevitably and only boil down to "look, our model converged". The point is, dark matter is not a theory, just a supposition that can always be "proven" with conveniently arbitrarily flexible models (no one can see it so it might be anywhere!).

There is no positive theory that has been brought out to explain dark matter, only mere deductive hypothesis about where it would be. Deduction needs a culprit so people keep reaching for different kinds of particles. Jury's out on whether they exist but keeps the experimentalists employed, so at least it's worth that much.

jboy55 987 days ago

There have been actually many positive theories trying to explain what particles dark matter could be, they've all been proven wrong.

To me dark matter came about from this,

"As we understand gravity we can postulate how galaxies rotate given an estimate on its mass, galaxies do not rotate like this"

In this statement there are three elements;

1) Our understanding of gravity 2) Our estimates of the mass of galaxies 3) Our ability to determine how galaxies rotate.

At one point in time this last 100 years, we had "solved" gravity with regards to our solar system, and we were finding so many new particles that #2 seemed like a great explanation. However, we are now left with no room in our understanding of particles, I think its time to look at the other elements.

Put it this way, if we had never observed the galaxy, but developed the standard model in isolation. Then we looked at the stars and tried to define gravity, I'm not sure we'd be so quick to introduce a new type of matter to define gravity.

nyssos 986 days ago

> There have been actually many positive theories trying to explain what particles dark matter could be, they've all been proven wrong.

Not at all. Ordinary neutrinos and MACHOs (black holes, rogue gas giants, extremely faint dwarf stars) are mostly ruled out. Supersymmetry is not looking promising but certainly not proven wrong. Sterile neutrinos and axions are very much live candidates.

> However, we are now left with no room in our understanding of particles

There's plenty of room. Quantum gravity is obviously the elephant in the room, but even aside from that, and off the top of my head: the standard model doesn't account for neutrino masses ,matter/antimatter asymmetry, or why the lepton masses are related, and it gets the magnetic moment of the muon wrong. The existence of physics beyond the standard model is certain. We just don't know what it is yet.

JumpCrisscross 987 days ago

Fair enough. I think it’s qualitatively different from SUSY, where I tend towards your conclusion of jobs programme masquerading as theory. (I’m much less convinced dark matter is a novel particle.)

ben_w 987 days ago

> no good reason is ever given as to why it could be matter and not anything else

The options are 1. matter; 2. energy; 3. new physics.

We'd expect pure energy to not stay still for long enough to do anything.

Lots of people are looking for new physics, and would be anyway even if it weren't for all the things that made us look for dark matter in the first place, because of the whole "relativity and quantum mechanics don't play well together, and neither is sufficient by itself" problems.

Dark matter requires something new, given it can't be explained by baryons or black holes, so it too needs some new physics — though as "matter" it would be in the particles-and-fields area rather than the how-does-spacetime-even area which the not-actually-matter solution would be.

nwallin 987 days ago

Dark matter is more complicated than that. It would be nice if it were as simple as you claim it is, but it isn't.

From the '30s through to the '70s, evidence was piling up that what we saw and what our models predict were incompatible. Zwicky's application of the virial theorem to a galaxy cluster, and various astronomer's calculations of galaxy rotation curves implied that either there was a bunch of stuff we couldn't see, or general relativity was wrong, or both. MOND was born in this era to explain that general relativity was wrong. (note when I say "wrong" I mean in the same way F = Gmm/r^2 is wrong: it's correct in the limit, but wrong in the extremes. Newtonian gravity is wrong at very high acceleration, and MOND implies General Relativity is wrong at very low acceleration) If science had stopped in 1985, you'd be correct: we couldn't tell the difference between dark matter as particles, (CDM, cold dark matter) dark matter as heavy dark objects, (MACHOs: brown dwarfs, black holes) or dark matter as a new gravity model. (MOND: modified Newtonian dynamics)

But science did not stop in 1985.

First and most obvious is gravitational lensing and the bullet cluster. This is well trodden ground, so I won't get too much into it: but the bullet cluster shows us that whatever dark matter is, it has momentum. Some MOND theories do predict something like that, and are compatible with the bullet cluster, other models are not compatible with that, and are falsified by the bullet cluster.

Second is baryon acoustic oscillations. (BAOs) In the few minutes after the Big Bang, the universe was, to a first approximation, a roiling sea of photons. There were electrons, protons, and the odd helium/lithium nucleus, but because charged particles interact via the electromagnetic force, they were being tossed about on the sea of photons. Baryons could not form overdensities because they were charged, and if anything thought about clumping up, the photons would scatter them. But baryons were not the only objects with mass: there was also dark matter. Dark matter could form clumps, and clumps formed by clumping dark matter would be able to clump normal matter. Eventually, the universe cooled enough that normal matter could clump properly, and at the moment the universe cooled enough to be transparent, the cosmic microwave background (CMB) was born. In order for the CMB to look the way it does, there are very tight bounds on how dark matter has to behave, and wouldn't ya know it, these bounds are compatible with the bounds on CDM in order to explain galaxy clusters and galactic rotation curves.

Third is the detection of ultra diffuse galaxies. These galaxies are remnants from a galaxy merger, which spilled off some of its normal matter but none of its dark matter, creating a galaxy with negligible dark matter. In other cases, these collisions create massive blobs of dark matter, but with little to no normal matter. These galaxies falsify MOND. For MOND to be correct, these galaxies cannot exist.

Forth is LIGO/VIRGO and the neutron star-neutron star collision a few years ago and the associated gamma ray burst. Many MOND theories predict that gravity travels slower than light. However, GW170817 shows that gravity travels at the speed of light. Some MOND theories are compatible with this, others are not.

In general, theories of MOND comes in two flavors: those that are compatible with the bullet cluster, and those that are compatible with GW170817. None of them, AFAIK, are compatible with both.

So if you want a MOND theory with no CDM, that's fine, but you have a number of hurdles to jump. You need to create a theoretical framework which is compatible with both the bullet cluster and GW170817, which nobody's been able to do. You need to show that ultra diffuse galaxies are a sensor or interpretation error; those galaxies are significantly closer or farther than currently believed. You need to come up with an entirely new mechanism that explains BAOs. It's not impossible, it is just extraordinarily difficult.

mr_mitm 987 days ago

Exactly. You can do all this and you'll end up with a super convoluted theory that basically says: the laws of physics have conspired to make everything look like as if there was dark matter.

The laws of physics (of the time) conspired to make everything look as if there was phlogiston, too.

Is chemistry "convoluted" to you? The concept of oxidation? Combustion? These are big words with lots of implications, are we sure they're warranted given how easy it is to chalk it up to phlogiston?

nyssos 986 days ago

> The laws of physics (of the time) conspired to make everything look as if there was phlogiston, too.

No, they conspired to make it look as if some substance was transferred between fuel and air during combustion, and between the air and the lungs during respiration, which is true. It happens to be absorbed from the air and not by it, but until and unless you actually devised an experiment to test that, there's no way you could have known. Anyone absolutely convinced of the existence of oxygen in 1600 was being just as unreasonable as someone absolutely convinced of phlogiston - they just got lucky.

The good reason for adding dark matter, that you say is absent, is one that is more a question of philosophy of science, in that adding more mass accounts for the observed behaviours without changing the known laws of physics.

The known laws of physics have been formed on math that checks out and is consistent with all our other observations, and has made many predictions that have checked out and even formed the basis for technology that we use every day.

The way science works is that we form mathematical models of physical behaviour, we test model against real world data, and if the model is consistent with reality, and predicts further behaviours that we then can test for, the theory behind it holds water and we have something to work from.

If you like, you can think of it as building trust in a model, having courage in a theory isn't a mistake, it's how science has been built. Of course finding the errors and new laws is important, but you have to conclusively rule out the established theory first.

This is how we got to Newtonian mechanics instead of firmaments, elements, worlds of forms and mythologies, and how we got to medicine instead of humours, phlegms, biles and alchemy.

Adding mass that can't be seen preserves the body of theory of the standard model and doesn't raise any questions of why GR/QM work correctly for things like GPS etc.

In other words, dark matter is an answer that doesn't require going backwards.

Saying that gravity behaves differently to what we previously thought means that the standard model is only coincidentally right or only right in particular places, and from there where does the scepticism end? Where do you even start unravelling the tapestry?

Think about it in a diagnostic analogy. If your patient is critically ill, and you don't know why, you will prioritise testing for conditions that fit the symptoms and can actually be treated/cured. Because if it isn't treatable, the truth of what caused it isn't that important.

Occam's Razor as an argument against dark matter, but saying that gravity behaves differently — when our theories do not otherwise predict that it should behave differently — is actually less simple than saying there is more mass than can be detected via EM interaction.

The other point I would make is that dark matter can explain most if not all of the otherwise problematic observations, which makes it preferable over modifying the laws of physics, because as I understand it, doing this doesn't account for all of the problematic observations.

Again with an analogy to medicine, it is less likely to be three unrelated, coincidental conditions in one patient than a single condition if both diagnoses explain the same symptoms.

To frame everything I've said in a medical analogy, we have essentially treated for the condition we think it is, and we're trying to figure out why the condition has presented differently to typical cases, rather than ruling out the diagnosis and saying it is something else entirely — because the treatment is working. That is, the empirical evidence we have suggests that our diagnosis is correct, but we don't know everything there is to know about the condition.

Our GPS works, gravitational lensing has been observed, gravitational waves have been detected, we power our homes with nuclear reactors, we calibrate our most accurate clocks based on quantum mechanics, and so on. Particles we then predicted would exist have since been detected.

The patient had a fever. We treated the patient with antibiotics, and they got better, so we have reason to believe it's a bacterial infection — we just can't see the bacteria in the blood work. So the next logical step is to think of what presents and responds like a bacterial infection but isn't bacterial, or otherwise speculate that we have discovered something that does this, rather than question whether we understand the human body or whether thermometers work.

If in trying to confirm this discovery, we find that actually we don't really understand the human body or that our instruments are broken, that's when we should start looking to re-assess the laws of physics.

The standard model has no useful purpose if we don't place some trust in it to find new things. If we threw out our scientific models every time we encountered something we weren't expecting, we wouldn't make any progress at all.

The only reason dark matter raises so many eyebrows is (a) the popular press just loves to pick at it because it's an easy target with great headlining when your scientists are saying the majority of the universe is "missing"; and (b) because the breakthroughs of today are framed as being incremental compared to the big eureka moments of the 19th and 20th centuries which saw us leap from Newtonian mechanics to GR and QM. But between Newton and Einstein et al, there were centuries of incremental refinements/improvements on Newtonian mechanics and early modern astronomy, so why is there such impatience because we haven't found dark matter in barely a hundred years?

By definition, dark matter is going to be hard to detect because the only means of detecting it is merely enough to suppose that it exists, i.e. it interacts gravitationally but not electromagnetically, so we can only detect its gravitational influence on celestial bodies. Of course, it's the odd behaviour of celestial bodies that led us to suspect that dark matter was a thing in the first place, so this isn't very helpful.

I should think that, in order to prove dark matter exists, we shall have to imagine an edge case of what an extreme concentration of dark matter would do to nearby celestial matter and how that might be distinguished from conventional phenomena. Easier said than done, the universe is full of bizarre phenomena, much of which can be explained by GR and QM, and any remainders probably defy any remotely intuitive reasoning.

Alternatively, we shall have to imagine what phenomena might occur in situations where dark matter is absent, and where something can be modelled mathematically as being conclusively due to a lack of dark matter i.e. if gravity were to be different to Newtonian/GR, the phenomenon would never be seen, we can then say with confidence that dark matter is real.

To confirm it beyond any useful doubt, I suppose we would need to create conditions under which dark matter would form and observe the phenomena that occur, or build some kind of instrument that can detect gravitationally as accurately as we can detect electromagnetically. Again, easier said than done, EM force has a fundamental particle that we know very well, while gravity ... well the jury's still out on that. The graviton even if it were a thing would not be a particle in the same way, you can't have a quanta of gravity, when gravity is more of an emergent property of the geometry of spacetime? You can do the math as though it has a force carrier, but this isn't something that you expect to be able to manipulate as a particle in application.

Anyway, I'm away on a pretty hefty tangent now. The point is that it's more constructive to suppose that there is dark matter, since alternative theories (a) also include dark matter, to a lesser extent and (b) do not account for the observed behaviours without in some ways failing to predict behaviours we know and understand with known physics.

nobodyandproud 987 days ago

> in that adding more mass accounts for the observed behaviours without changing the known laws of physics

I disagree with statement in that I feel this is an incorrect interpretation of what transpired with physics.

Classical physics, modified and tweaked over the centuries, worked well and it’s still valid for the domains where it was already conceived and tested for.

The cracks in the model formed when we pushed experimental boundaries.

Very high speed and extremely tiny were both new, but most (all?) of the vetted modern models will simplify down to classical physics when in every day conditions.

The new boundary condition is galactic scale mass and distance but with mostly (?) non-relativistic speeds and probably subtle GR gravity conditions.

MOND? Darkmatter? It’s good science explore all avenues, not shutdown a discussion until conclusive evidence and lack of rebuttal shows otherwise.

Otherwise it’s not science. These days, I’ve started to suspect that it’s not scientists that have such a black and white view and certainty.

> It’s good science explore all avenues, not shutdown a discussion until conclusive evidence and lack of rebuttal shows otherwise.

I couldn't agree more. I don't think anyone should claim to be certain about any of our understanding of the universe. I just say that it isn't very useful to think that way, we make more progress if we have the courage to trust in a theory and dare to be proven wrong, than to go back to the drawing board when we get stuck.

nobodyandproud 987 days ago

For uncertain things, as long as the courage extends to all feasible (meaning, not outright disproven like flat-earth…) models then we’re in agreement.

What bothers me generally, is how one speculative theory dwarfs others when the scientists themselves will admit that there’s far more wiggle-room.

This has implications in funding and brainpower, hindering progress in the long run.

Yes, in case someone asks: The contrarian side where alt-theories include the outright disproven is also worrisome; but in HN at least everyone seems pretty bright and attentive to good science; far smarter than yours truly, certainly.

The flaw in this reasoning is ironically a philosophical (epistemological) one: by what authority is it said we "know" the laws of physics? We "believe" theories, even go so far as to sometimes call them "laws", but as we've seen with Newtonian interpretations (the "law" of gravity) they can obviously be superceded by more elegant, positive (not merely deductive) theories, i.e., general relativity. Who is to say our current understanding is the correct or best one? Granted, it's a good place to start for the experimentalists, but for some reason theoreticians have also drank the Kool-aid rather than honestly examining the other proposed theories.

> The known laws of physics have been formed on math that checks out and is consistent with all our other observations

You will of course note that "all the other observations" conveniently reside in the limit of high-mass-density regions of spacetime, where other theories also expect the current best theories of physics to hold. Where the confusion still lies is in the low-mass-density regions. At least other theories posit some explanation besides "there's still mass it just exists in other dimensions". Sounds like sci-fi crackpottery when put so plainly, but I'm sorry to say this characterization is accurate enough for our needs here.

> where does the scepticism end?

Strange application of slippery slope fallacy. It obviously ends where the theories still hold, i.e., high-mass-density regions. This is IMO enough of a response to most of your "philosophical" arguments.

No one's denying that particles exist. I'm only arguing for a theory that actually posits something besides "oops there's a gap". You've articulated a reason why dark matter is offered but it is nothing more than a deduction about where matter would be should it exist. I swear I'm going to have to spin up some cycles on the cluster to fit dark matter models on a geocentric universe to get you people to understand the non-reality of any dark matter paper.

Gosh I love being mansplained on this site by people who obviously have no personal experience with this stuff. Really gets me going.

Edit to reply to your edit:

> when gravity is more of an emergent property of the geometry of spacetime?

Emergent gravity and dark matter are incompatible theories in their usual forms, though there's probably ways to mash them together into a chimera. I suggest reading more into emergent gravity -- entropic gravity is interesting but still in its early stages. I'm not advocating for any particular theory, just humility from those who repeatedly insist that dark matter is already correct and we just need to find the matter.

> since alternative theories (a) also include dark matter, to a lesser extent

No they don't.

freedomben 987 days ago

> Gosh I love being mansplained on this site by people who obviously have no personal experience with this stuff. Really gets me going.

I say this in the hope that it's constructive. You should try not to sink to this level of rudeness and assumption of other people's motives/situation/perspectives. Not only is it a rude assumption about something that (to me) looks to be a good faith attempt at conversation that also clearly took some time to compose, but it's an emotional response that shows that you take it personally and emotionally when you're challenged. I'm not implying that GP is an idiot (I'm too ignorant on this subject to know), but I've been challenged in subjects where I'm well versed by idiots many times and I tend to react the same way that you did. It can be enraging (especially when surrounded by down votes and social/group reinforcement from other idiots), but you immediately lose any power of persusasion with other people when you stoop to that level rather than keeping on the high road and keeping it factual/scientific. IMHO you're rarely if ever going to convince the person you replied to, but the third party observers are often much more persuadable. They're the people I mainly try to write comments/replies for.

I'm not responding to the challenge to my points. I'm responding primarily to the infantilizing tone explaining how science works, and especially triggering is how inaccurate it is yet delivered with such confidence. It often feels like commenters want to cosplay intelligence and see how far they can get -- in other words, there's a lot of bullshitters on this website.

If others cannot judge an argument on its merits, that is not really something I can control. I understand your point about rhetoric re: persuasion I'm just resistant to playing civility games in what should be a facts-based discussion.

I acknowledge I am impatient with those who refuse to offer good faith responses. In my opinion such good faith would mean engaging with the facts of the matter not running through a phil.sci. 101 lecture, however sincerely.

Thanks for your engagement. Your point about third parties is a good one, one I keep forgetting and re-learning.

It was not my intention to trigger anyone. I only wanted to say why dark matter is given the time of day. If I came across as condescending or as a know-it-all, I apologise, it was not what I wanted at all.

I also must apologise for my use of the term "emergent property" regarding gravity — judging by your response, I seem to have alluded (unintentionally) to a whole other theory of gravity; I only wanted to say that gravity itself is the curvature of the geometry of spacetime rather than a force in the conventional way it is described.

Also, regarding alternatives still requiring dark matter, it is my understanding that MOND and its derivatives explain galaxy rotation curves but not other phenomena that dark matter is purported to resolve (galaxy cluster formation/structure, gravitational lensing, CMB). If I am wrong about this, I would welcome correction. On the other hand, if your comment simply meant that there are alternatives to DM and MOND that require no DM, fair enough, I should have been clearer and said that some of the foremost competitor theories still require DM.

But I stress again, I am not fighting DM's corner or saying that alternatives are wrong. My stance on it is irrelevant, and I have no more belief in it than any other explanation, belief is irrelevant and doesn't enter into the matter. I was just saying that I understand why a theory that inflates mass arbitrarily, and understandably ruffles some feathers as a result, is given any credence at all.

Personally, I understand your frustration with DM, it does not seem like very good science to let unexpected or inexplicable observations make us simply add parameters without making further predictions to test if that's the right thing to do. Does seem like we're manipulating facts to fit the theory where we should be altering the theory to fit the facts.

Since DM is a substance that, for all intents and purposes, defies detection by any means at our disposal, it makes no further predictions, it just lets us push the square block into the round hole — what we should be doing is finding the square hole.

nyssos 986 days ago

> Since DM is a substance that, for all intents and purposes, defies detection by any means at our disposal, it makes no further predictions

This is completely untrue. All serious dark matter candidates are observable. For example:

- MACHOs should show up in gravitational lensing surveys. We did the surveys, they didn't, MACHOs were rejected. Exactly the way it's supposed to work.

- Axions convert to photons in sufficiently intense magnetic fields. ADMX has ruled out part of the parameter space for axions and is undergoing upgrades to test the rest of it.

- Other WIMPs still interact via the weak force, and therefore with nucleons. There are many experiments looking for WIMP scattering. A few of them have gotten signals but not enough to be convincing.

Dark matter candidates are not just "mass with no further properties" sitting out there to make the model fit. They're proposed extensions to the standard model (which is nothing but proposed extensions to quantum electrodynamics which ended up working out), and therefore very tightly constrained by the standards of any other scientific field.

At no point did I say the laws of physics are beyond question or doubt, and neither did I say dark matter is correct.

I just said it makes more sense to give DM priority because what is the point of having a model if you don't start from it.

Also why are you being so hostile towards me exactly? I didn't express any love for dark matter or the standard model, and I am not a fan of perpetuating a status quo or any form of academic dogma.

All I said, quite unsuccessfully it would seem, is why people think its likely for dark matter to be there — because if you add in "invisible" mass with existing laws of physics, you get something that looks like what we see through our telescopes. I think many would prefer to suppose that there is non-EM-interacting mass (a lot of it apparently), than there being as yet unknown behaviours of gravity/spacetime geometry, which we like to think we understand pretty well.

Though I agree that we don't understand the universe as well as we like to think; and that the universe is not intuitive at all most of the time; and a preference based on how intuitively likely something seems is irrelevant to what the truth will turn out to be.

Edit: just FYI, I am not downvoting your responses by the way. I am not bothered if you dislike me or disagree with things I have said, though these two things should be distinct from each other.

I'm nobody and I didn't seek to "mansplain" anything whatever this term is supposed to mean. I was just talking, always happy to debate, something I thought people came here to do. I won't make the mistake again.

forgotpwd16 987 days ago

>"there's still mass it just exists in other dimensions"

What? Dark matter is there. Leading models consider it to be particles that don't interact with e/m field and interact weakly with gravity so is undetected in low-density regions. That doesn't make it other-dimensional.

>Sounds like sci-fi crackpottery

Like the prediction of particles in standard model? Is Higgs boson that went undected for 50 years sci-fi crackpottery?

There're issues with dark matter but it also (sadly some may say) happens to be the best explanation of the observed phenomena since all alternative models fall (plus although simpler at first sight quickly get more complex) in more ways than dark matter.

>I suggest reading more into emergent gravity

Emergent (either entropic or induced) gravity has nothing to do with the comment. It's obvious what GP meant. The correct should've been intrinsic property but this is nitpicking.

Being able to fit models is very far from being able to say "it's there", especially the kinds of models that are being fit. See my responses to sibling comments. "Leading theories", especially given the human impulse toward consensus, got the charge of the electron wrong for a long time before the culture readjusted and decided to look fresh at the problem with new experiments.

> happens to be the best explanation

The people who repeat this in popular science are just repeating what they hear from academics who, surprise!, have invested their entire career and reputation in the scientific community on that being true. Science demands more skepticism and interest in the truth than parroting status quo. It's also technically true only if you assume that explanations built on fitting extremely flexible nonparametric models are theories, but that doesn't seem like a mindset that's very interested in those theories representing truth per se.

> Like the prediction of particles in standard model? Is Higgs boson that went undected for 50 years sci-fi crackpottery?

Transparent straw man. Barely worth acknowledging.

raattgift 987 days ago

> interact weakly with gravity

It would be better to rearrange this to "interact gravitationally, and maybe non-gravitationally at the scale of the weak nuclear force".

In the standard cosmology \Lambda-CDM, cold dark matter ("CDM") is allowed to interact at the scale of the weak nuclear force (wnf), which may allow participation in nuclear interactions. This motivates the search for direct detection in various track and scintillator experiments, where a WIMP (weakly interacting massive particle) could take recoil energy from an atomic nucleus, the latter leaving behind a trail of charged particles and/or emitted photons. There are of course other hypothesized interactions between WIMPs and normal matter which do not literally engage the wnf, but instead have some new interaction at no more than the same energy scale ("weak scale"). And for completeness, there are CDM models which have stronger but rarer individual interactions which average out to the weak scale (or effective collisionlessness) across volumes comparable to the size of galaxies or galaxy clusters.

(In the standard cosmology what matters is that the equation of state for dark matter is 0 or close to it so that expansion dilutes it away like cold baryons; in structure formation and galactic dynamics it's more important that most dark matter is in a halo outside the luminous structure. Both are incompatible with decays or collisions which emit relativstic particles (w > ~ 1/6 leads to early evaporation of overdensities; galactic dynamics is less tolerant of (non-radiative) clumping and other mechanisms which concentrate/gravitationally-collapse halos).

> interact ... with gravity

In a system of coordinates that absorbs linear and angular momentum, all matter -- dark or otherwise -- interacts gravitationally in proportion to its mass (or energy-density).

In General Relativity, this is the universality of free fall, which descends from Newton's Law of Universal Gravitation.

Unless we modify or abandon General Relativity, a dark matter particle and an alpha particle occupying the same starting point and having the same initial velocity will follow the same trajectory together forever barring some interaction (examples: intrinsic: one of the particles decays emitting radiation, or there is some weak scale interaction between them that imparts a recoil on one or both of the initial alpha or DM; extrinsic: scattering of a photon off the alpha, or the alpha captures electron(s) imparting a recoil -- a force that shoves the alpha onto a different trajectory, a non-gravitational acceleration). This is the weak equivalence principle (WEP), <https://en.wikipedia.org/wiki/Equivalence_principle#The_weak...> from a somewhat Fermi-Walker perspective; the "weak" in WEP is unrelated to the weak nuclear force.

Finally, dark matter and baryonic matter (and all other matter and radiation) interact the same way with the curvature of spacetime. If dark matter and baryons interact non-gravitationally at all, the (averaged) interaction is at no more than the weak scale. \Lambda-CDM does not require any non-gravitational interaction between CDM and baryons.

> so is undetected in low-density regions

Density of what?