What would be the implications for cosmology theories if observations ruled out the existence of collisionless Cold Dark Matter (CDM) – which is a key ingredient in Lamda-CDM concordance cosmology? Are there other viable DM candidates that have not already been ruled out? Would we simply increase the portion of the universe that is dark energy? Would gravity theory need to be modified? Would the Big Bang Theory be in trouble? Are there other consequences?

Note that I’m not interested in this thread turning into a discussion of evidence for or against CDM. Given that for over 2 decades most cosmology research adopts CDM as a key ingredient in the universe, I’m wondering how far reaching the impact would be for those models if CDM was ruled out?

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

No; I don't know, but I would imagine it would be pretty darn awful. But; I do question how you could visually rule it out, rather than finding some directly conflicting evidence. Even if it were just your wording, wouldn't that conflicting evidence be the driving force of any problem?

btw: why the font?

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Numbers are not case sensitive. (me)

Wouldn't be the first time that theories are stricken and re-thought.

....that's what science is about, establishing and proving theories or coming up with a better idea, not holding onto dogma.

We're just interpreting the universe, not making the rules.

There are ways to rule out CDM.

What is wrong with the font? Is it real big on your screen? I've taken to typing all e-mails and BAUT posts in MSWord first because I've too many times lost everything I typed when the computer freezes up after I hit submit. When I pasted the OP into the BAUT software it came up very small so I enlarged it. It appears normal sized on my computer, but I wondered if it would be really big on everyone elses's screens. I'll fix it.

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

Smallish and hard to read, and this from someone real fond of Times New Roman.

If CDM is ruled out, things get really difficult. I have seen claims that you can get around the whole DM mess with GR, but I cannot judge the merit of those claims, and I would have some real reservations about them since the majority of the mainstream people support (C)DM.

(I removed the SIZE and FONT tags ... like others I had difficulty reading the post).

Personally, I'd love to be part of the team that wrote the paper reporting the observations and analyses that historically came to be known as the announcement of the death of non-baryonic CDM!

Given the enormous range of observations etc that point to CDM, I'd say the realisation that 'there ain't no CDM' would come slowly, in dribs and drabs, spread out over many years, so when the day finally dawned it would be completely anti-climactic ... cosmological models would have long since adapted and adopted the deltas.

AFAIK, the leading borderline candidate that has not yet been robustly ruled out is isolated black holes, if they are in some sense primordial. However, I think the most exciting possibility is a modification to GR, or a completely new theory of gravity altogether. The implications of such a new theory may make for wonderful speculative discussion over a glass or three of your favourite wine, but without any constraints on the nature of such a theory, I'd say it's impossible to conclude anything about a Big Bang becoming history (or not) ...

Apologies before hand for going OT...

Yes; I tried to indicate that. It was your wording that I had a hard time with. It implied you could see that it wasn't there, rather than, you find other things to see that it wasn't there (by conflicting observations).

The bolding just made it somewhat imposing...

The computer or the session/application? Yes; that can be a pain.

Yeah, and back to a smaller arial instead of verdana. I would suggest removing all formatting and using default. Otherwise, you never know what's going to happen to someone with a different font set.

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Numbers are not case sensitive. (me)

It would get dropped just like how the observations added it in the first place.

Dark energy and dark matter are pretty much orthogonal: the evidence for one is generally completely unrelated to the evidence for the other (see the confidence level curves for BAO, SN and CMB observations). I think it would depend strongly on what this "observation" is that "rules out DM," and what scale it was on. Could you give us a hint about what kind of null observation you are thinking about? Finding a lone galaxy that doesn't appear to need dark matter wouldn't be enough. The only way I could think of to "rule out" dark matter is by coming up with a cohesive framework that explains all the observations as well or better...

In that regard, there are certainly people currently working on various varieties of MOND, but so far none of them do the job as well as CDM. The best one available (Milgrom's, I think) requires neutrinos that are more massive than the current experimental limits, I believe.

And I'll echo Nereid's comments and even expand them: most astronomers would love to be part of the group that "rules out" dark matter. But the observation, or new theory, would have to be very solid in order to get traction.

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"What do you care what other people think?" -- Richard Feynman
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Depends very much on the nature of these hypothetical observations!

Also ditto parejkoj on the distinction between dark matter and dark energy.

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In reply to dgruss23’s comment & question:

1) I thought there was not a strong driver from the standard model (physics) for a “dark matter” particle to exist, from the standpoint of theoretical particle physics. i.e. It is my understanding that theoretical particle physics (standard model) would need to be revised to allow for dark matter to exist.

2) I thought the earth based dark matter detection experiments have become more sophisticated and are moving toward ruling out most of the reasonable possible candidates for dark matter particles.

3) I noted that there are observations such as compact massive early galaxies that need some mechanism to unpack, which dark matter does not explain. Also I copied that paper that noted numerical analysis with CDM does not agree with observations for spiral galaxies.

Based on 1 through 3 it would be reasonable at this point in time to start looking for another explanation.

4) From what I have read MOND does not explain the observations. As far as I know there is no theoretical reason to propose MOND from a GR or any other theoretical basis (standard physics).

5) There has been an attempt to explain specific anomalous galaxy cluster motion (like the great attractor), due to intrinsic redshift by Bell. Intrinsic redshift would not however explain the anomalous lack of change of rotation speed with radius, for spirals. (Also a mechanism is required to explain intrinsic redshift which is a separate issue.)

6) There are other anomalies that could be examined to look for clues to help find the underlying mechanism. There does seem to be connections between anomalies. Dark matter is a bit of pain if it does not exist, as that theoretical idea complicated and slowed down the solving of the problem.

7) The place to start would be to try to explain the observations using standard physics rather than postulating new particles or dimensions. As I said there is a bit of a risk to change the laws of physics to explain an observational anomaly.

Earth-based dark matter experiments have only barely begun to scratch the parameter space of the reasonable standard model extension candidate particles. And several of the possible standard model extensions (actively worked on for many years, and with good reason) naturally include particles with dark matter-like properties.

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"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

Thanks for fixing that!

I think you're right the evidence would come in in dribs and drabs, but I think researchers are so highly specialized that the dribs and drabs might slip under the radar of many researchers. So it would be just as likely that a critical mass of such evidence would need to be reached and then it might be quite climactic after all.

I know that MOND is "modified gravity" but there are others although I've never had a chance to look at them. I've just noticed that almost daily on the astro-ph postings there is a "modified gravity" paper.

That's certainly why I ask the question. I'm wondering how far reaching the effect on theory would be if CDM was ruled out observationally. It would obviously impact the current prescription for the lamda-CDM concordance model since 23% of the density of the universe comes from CDM in that model. But can an alternate set of parameters be made to fill that 23% gap and thus leave the rest of the successes largely intact?

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

Agreed, but when you put them together with other observations and derive the parameters of the concordance model from all that, I'm wondering how far ranging the impact would be if we suddenly had to remove the CDM component from the model - as I asked Nereid.

I've read numerous papers that demonstrate problems for collisionless CDM on galaxy scales. Some of the problems stem from failures of observed DM distributions in galaxies to conform to the NFW simulations. Other observations require a tight coupling between the distribution of the DM and visible matter which is very difficult to account for with a collisionless DM particle such as CDM.

I'm so busy these days that I don't have a lot of time to link to and explain the relevant results and conclusions from the numerous papers that discuss these issues. But I'll try to find some time soon. In the meantime, I'm still curious to find out what people know, think, speculate would be the possible resolutions to results if such observations did in fact rule out cold dark matter.

Note that in these discussions I do make a distinction between cold dark matter - which is a collisionless particle with properties that results in certain expectations - and other potential DM candidates. So in other words I do not equate a conclusion that there is no cold dark matter to mean the same thing as a conclusion that there is no dark matter.

That depends upon what scale you look at. On galaxy scales MOND is superior to CDM, but on galaxy cluster scales CDM seems to have more success than MOND - although MOND advocates might disagree.

I think you're right on that. The last few years they required neutrino masses very close to the limit allowed by observations. It is possible that more recent observations have lowered the upper mass limit for neutrino's to something less massive than MOND requires - but I'm not positive about that.

If most astronomers would love to be part of such a group, they do have avenues that could be examined more closely - I'll try to provide references ASAP - something that will require much more time than I have at the moment. But I'm still interested in thoughts on the other question. I really don't know how far reaching the impact would be if CDM was ruled out - perhaps it would be a minor adjustment.

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

(1) There would need to be some other explanation for the observed orbits in spiral galaxies.

(2) There would need to be some other explanation for the observed dynamics of galaxy clusters.

(3) There would need to be some other explanation for the weak gravitational lensing observed exterior to the visible mass in the colliding Bullet Cluster.

(4) There would need to be some other explanation for the structure formation in the very early universe.

(5) There would probably need to be other explanations as well.

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Everyone is entitled to his own opinion, but not his own facts.

There's always a danger of something like this; to add an example: the number of active researchers drops to just a dozen or so.

But even more problematic is that there are no guidelines whatsoever for us, today, to estimate where and how and how fast what in the (far?) future will be called something like 'the history of CDM, the 20th/21st century phlogiston'.

Maybe it will be an obscure result from a now-dismantled particle accelerator that a bright student works on as a summer project that kicks off a quick follow-up experiment that immediately opens the door to a whole new line of research that results, a decade down the track, in 'the death of CDM'?

Maybe it will be a perfect example of a scientist's worst nightmare - 1 and 2 sigma deltas reported in dozens of widely scattered areas, most of which no one followed up on, and the ones which were followed up all turned 'null' due to a conspiracy of mistakes, unidentified systematics, and so on?

Amazing isn't it? And on many days there are considerably more than one new preprint!

Of course it's a cline, but the distinction between a modification to GR and a completely new theory I am making is important in terms of its implications for the OP: a modification to GR would be less likely to overthrow CDM than a completely new theory, if only because the observational evidence for CDM is found in so many environments and over such an enormous range of sizes, densities, etc.

Myself, I think it is great to speculate, but without the faintest clue as to how CDM gets to disappear, observationally, there's the same zero basis to answer this last question.

For example, in cosmology, the "23%" comes at the end of an extraordinarily long chain of logic, observations, and just about all parts of the physics textbook. Now perhaps a new theory of gravity comes along and when inserted into the chain the 23% disappears, and we all go 'oh! ah! how sublime and elegant it all is!!', and your question gets answered thusly: 'the new concordance cosmological models are much more self-consistent and robust than the LCDM ones were, yet nothing changed except that {new theory of gravity} replaced GR'.

Suppose it turns out that non-baryonic CDM is quite heterogeneous:

* some is just plain old baryonic DM that was hithertofore underestimated

* some is an interesting population of black holes from shortly before the time neutrinos streamed free (the neutrino equivalent of the CMB's 'surface of last scattering')

* some is a quite different population of black holes from the era of Pop III stars

* some is an exotic form of baryonic matter that formed before/around the time of nucleogenesis (stable, supermassive neutral quark nuggets perhaps)

* some is truly non-baryonic CDM, but of many different kinds (sneutrinos perhaps, and axions, and ...).

And suppose that these different kinds of CDM are distributed quite differently among galaxies, clusters, and otherwise (but collectively they sum to mimic the homogeneous class we 'see' today).

If this turns out to be, then no single team will ever write a 'death of non-baryonic CDM' paper (or even papers); instead a hundred separate papers will be written, each partially solving a part of the puzzle ...

Me, too, even with my rather large, two-full-page-capable monitor.

Would the OP consider posting in the default typeface?

Thanks, from all of us who're over 32...

Nereid. As I've just finished summer-stocking my wine rack...feel free to stop by for a few glasses and I'll give you the lowdown on my prior conversation with KenG on my reluctance to announce my pet theory years ago as it predicted a necessary modification to Keplerian rotation curves and GR , and, an as yet unidentified-second-order solar neutrino deficit. pete

...and I'll watch my "sometimes flippant" attitude..lol

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A third rate theory forbids.
A second rate theory explains after the fact.
A first rate theory predicts.
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I think Nereid's comment that the evidence will come in dribs and drabs is most likely correct. A couple examples come from simply more accurate mathematical analyses or more accurate measurements.

Ken Nicholson's calculation discussed a year or so ago on this forum showed that spiral galaxies can be represented with all the mass in the disk and produce the proper rotation curves with about 30% less mass. This is an example of reduced dark matter using a more accurate analysis (Note : Newtonian Gravity only).

Currently accepted galaxy cluster mass calculations result in about 80% dark matter and about 20% normal baryonic matter. As non redshift distance measurements to cluster members become more available, interesting results follow. For the Coma Cluster (considered sufficiently virialized), data for nonredshift distances to a number of galaxies in that cluster show a distance (D) of about 85 Mpc, as opposed to the redshift distance of about 100. The 80 - 20 mass ratio is computed from the 100 Mpc value.

The cluster mass is proportional to r*v^2 from the Virial Theorem, where r is the distance from the cluster center to an individual galaxy, and v is the galaxy peculiar velocity. Since r is directly proportional to D, if the nonredshift value is correct, this results in an immediate reduction of the cluster mass estimate by 85/100.

The peculiar velocity is the cluster mean value - the individual galaxy value. Since these are calculated from H0*D (Hubble constant times distance), the velocity values are also reduced by 85/100.

The combined result of the above is that the estimated galaxy mass is reduced by (85/100)^3. Doing the math results in a reduction of the cluster mass to about 61% of the redshift based value, and thus the dark matter is reduced by almost 50%. This is an example of reduced dark matter using more accurate measurements.

I'd like to repeat something dgruss23 said in the OP:

"Note that I’m not interested in this thread turning into a discussion of evidence for or against CDM"

Can I ask that we keep this thread focussed on the questions in the OP?

From my POV, the only question that is not hypothetical - and so all answers must be speculative - is this one: "Are there other viable DM candidates that have not already been ruled out?" I took a shot at answering this; does anyone else want to have a go?

Sorry. I guess my interpretation was that the "null hypothesis" was a viable candidate that has not been ruled out, i.e., more accurate measurement/analysis may eliminate the need for other candidates. Wanted to back up my statement with data.

In reply to Nereid’s question:
I would suggest that charge unbalance is a possibility. I am not prepared to promote that as an ATM at this time, but I am thinking about it. i.e. How would you know if there was or was not charge unbalance through out a galaxy? Would charge unbalance affect redshift for the astronomical object in question? Bell claims to have found evidence of intrinsic redshift anomalies for specific clusters of galaxies, in addition to quasars. I am thinking about the finding that BH have a maximum size of around 10^10 solar masses. Also spiral galaxies have a maximum size. I am looking at the differences between elliptical and spiral galaxies to see if it could be explained. I also notice there is a finding of anomalously high temperature intergalactic gas in the center region of spiral galaxy clusters. An elliptical galaxy is often found in the centre of the cluster.

There is this finding of a spiral galaxy that does not have the anomalous rotational speed variance with radius, as do other spiral galaxies. I would be interested in seeing a picture of the galaxy in question. Does it have any morphological differences compared to other spiral galaxies? If it does, perhaps the morphological differences could provide a clue to what is the underlying general mechanism.

http://space.newscientist.com/articl...tronomers.html

I think Nereid's question is interesting as it seems to be a logical strategy to approach the problem. What are the possible alternatives? Are there other unexplained anomalies that could provide a clue to an alternative mechanism?

On many days there are more than one - tonight I had to get to the last entry on the new listing, but as usual, there was a modified gravity paper.


That is an important distinction. Many modified gravity theories attempt to account for observations assuming the existence of CDM. I'm looking at this from the hypothetical result in which observations accumulate that rule out collisionless CDM rather than someone developing a better gravity theory that eliminates the need to infer CDM from the observations. How then would cosmology need to be modified if there was no CDM?

I would disagree that there is zero basis to answer the question. Imagine a scenario where the reason CDM is ruled out is that every possible CDM candidate is eliminated at all relevant masses via the various experiments attempting to detect these particles (forgetting all the difficulties with actually ruling out DM this way). If such an outcome occurs, we still need to explain the observations that originally led to the inference that CDM is a viable explanation for those observations. So again if CDM is ruled out, how does that effect the prevailing cosmological model. We can explore that question without CDM being definitively ruled out.

But I'm getting the sense that there is not much interest in the question because a specific case against CDM has not been brought forward here. I'll see what I can do to organize some relevant references from my files that outline such a case.

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

I think one problem with your suggestion is due to the number, and type of observations in favor of dark matter (there's a BAUT thread listing many of them). If there really were "dribs and drabs" of observations that "ruled out" dark matter, it would have to be across many different fields, and the results really would have far-reaching implications.

A big problem is that the WMAP results (indirectly) put strong limits on the "flatness" parameter, Omega-k. And the flatness and matter density are related (see the "Braneworld" plot in the lower-left of the SCP Union Compilation), so if you reduce Omega-M while keeping Omega-k fixed, suddenly other observations don't make sense.

But I don't think there is any way to have Omega-M at the currently accepted value (~0.27 times the critical density) without dark matter. All the current MOND models require some form of dark matter to match observations. And the structure formation observations (BAO, LSS, etc.) don't make sense if Omega-M is large and entirely baryonic.

And, I can't really ignore the difficulties associated with eliminating "every possible CDM candidate," since we're presently just scratching the surface of possibilities. It will be quite a few years before we've made a significant dent in the parameter space. On the other hand, if the generation of experiments that are coming online now don't find anything, cosmologists might start to get a bit worried. But we've mostly left the problem to the particle physicists now: we've provided some of the parameters to look for, now they need to go find it!

In many ways, it would be easier to rule out dark energy. Here are two papers talking about the expected observational signature were we to be near the middle of a ginormous void (~1GPc scale). This requires abandoning the Copernican Principle, which is near and dear to cosmologists' hearts, but it seems a reasonable approach. Plus, they include details about how it could be ruled out with supernova or the kinematic Sunyaev-Zeldovich effect, and those observations should happen in just a few years.

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"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

Are you saying here that IF WIMPS, that were/are completly "Made Up" particles, with NO observational evidence, are ruled out, that Non-baryonic Dark Matter (Collisionless) should still Not be ruled out?

From everything that I have read and studied it does seem pretty convincing that mainstream has got it right that there is Not enough baryonic matter to be found to be able to account for the rotation curves.

The Cluster dynamics is totally dependent on the virility of the "Viral Therum" and the "Bullet Cluster" is a whole "Nother Story" as it is "Colliding Gas".

Yes, like the 200 dwarf galaxies per spiral galaxy result.

AND, speaking of "Simulations" (SIMS)...that is where 98% of the 'observations' are coming from!

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

Personally, I find this quite humerous!

IF, WIMPS are ruled out, THEN there was never ANY observation, that ruled them in! WIMPS are a 'completely made up' ad hoc 'particle' that there has been NO observational evidence for.

AND, IF they existed, then they should have been 'observed' before Neutrinos were ever 'detected' as they are 'slower' and 'more massive'.

Nereid, IMHO, you continually "Way Overstate"/misuse the term 'observation'(s).

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

There are different types of non-baryonic dark matter - The most familiar type is cold dark matter. There is also warm dark matter, self-interacting dark matter - and neutrinos have been shown to exist, but the particle masses are not yet known.

Warm Dark matter and self-interacting DM have their own problems and therefore most papers focus on CDM.

In the baryonic category, molecular hydrogen may still be a possibility although I believe Pfenniger and Colmes have not had much to say about that lately. Their original work on the question was in 1994.

So I'm talking about observations that would rule out DM being a collisionless particle such as CDM. I'm not going to anticipate every possible type of DM that might be derived from particle physics - including non-baryonic DM that is collisional with normal matter. However, that was one of the points of my question - if CDM is ruled out what are the options. Is there a DM candidate that is non-baryonic but collisional which has not been ruled out?

MOND researchers would disagree with that. MOND matches the rotation curves of spiral galaxies using observations of the light profile, hydrogen gas measurements and reasonable M/L ratios. MOND does not need extra mass to explain spiral galaxy rotation curves. Where MOND needs extra mass is to explain galaxy cluster dynamics - but even here the mass needed is only ~twice the observed mass. But MOND isn't the topic of this thread.

I'm organizing the articles from my files (as time permits) and hopefully soon will provide links the the papers relevant to this discussion.

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

I think there's a bit of a confusion in this thread about what "CDM" is, which in turn reflects the independent yet overlapping nature of components of the relevant models and classes of observation.

Let's have a look at a few, shall we?

First, gravitational "lensing". This is an application of GR, and (in a highly simplified form) says that mass 'bends' or 'deflects' or 'lenses' light (or photons or electromagnetic radiation); it's just what's behind the first experimental/observational success of GR, the displacement of various stars near the limb of the Sun (from their expected positions) during the 1919 total eclipse*. While the details of lens reconstruction can get very messy, and while there are some degeneracies, at a general level we can say that an analysis of good observations will conclude: IF GR THEN {insert estimate of mass}. This technique has been used to estimate the mass of various clusters (both rich and not so rich) and galaxies (of various Hubble/morphological types).

How much of the mass estimated by gravitational lensing techniques is 'dark mass' (or 'dark matter')? At one level that's a very easy calculation to do: count all the photons from the object being studied, develop a model of what sorts of things produce photons in such an object (e.g. stars, dust, gas), apply number crunching and statistics, and out pops an estimate of the mass in the object which is producing the detected photons ... the 'DM' in the object is then just the difference.

Note that this conclusion concerning the amount (and distribution) of DM says nothing about its nature ... in particular, it says nothing about whether it is 'hot', 'warm', 'cool', or 'cold'; it also says nothing (directly) about whether it is 'baryonic' or 'non-baryonic'.

Can gravitational lensing be used to estimate mass in cosmological observations? In particular, is there a gravitational lensing signal in the CMB? Yes, there is. Or, more conservatively, there should be but it hasn't been observed unambiguously yet. It's called the Integrated Sachs-Wolfe effect.

Second, "baryons". This is a shorthand for atomic nuclei, from H to U and beyond; strictly speaking it does not include the corresponding electrons (which are leptons), but for all practical purposes it does. Potentially, all (astronomical) baryons are 'light' matter (i.e. not 'dark matter') - no matter in what shape or form, all atoms, ions, molecules, etc, etc, etc either emit light (I'm using the term generically, to cover all electromagnetic radiation) or absorb it.

By using 'telescopes' that cover the whole electromagnetic spectrum, and raiding the physics textbook for how (astronomical) baryons interact with light, we can build models that successfully account for all the photons we see (and those we don't see, e.g. in absorption lines) ... and thus can estimate the total mass of baryons in the observable universe (and its distribution; with appropriate error bars/uncertainties, of course).

If there is a difference between the estimated mass of an object (a rich cluster of galaxies, say) and the estimated mass of the baryons in that object, we can give the type of matter which makes up the difference^ a name: 'non-baryonic matter' or 'non-baryonic mass'. Note that such matter is 'dark', as a logical consequence of how the baryonic mass was estimated.

Naturally, there are a whole laundry list of caveats to enter; it turns out none has any significant impact on the above. For example: the black holes that are the 'invisible' companions in various x-ray binaries - first these have such a trivial total mass (compared to that of even their parent galaxy let alone the group or cluster they are in) as to not even show up in the 3rd decimal place, and second they were once baryons (this is important for the BBN case briefly discussed below).

Third, Big Bang nucleosynthesis (BBN). In modern cosmological models, some stable light nuclides were formed during a very specific time period in the early universe - hydrogen, deuterium, helium-3, helium-4, and lithium-7 - and their relative abundances (in unprocessed material), interpreted within these cosmological models, tightly constrains the number of baryons in the observable universe. Of course, for the purposes of this thread the logic should be reversed, but the conclusion is the same - we can get an estimate of the total baryonic mass of the observable universe, and if the estimated mass is greater than that from baryons, then the difference can be called 'non-baryonic mass'. Note that in this case there is no directly necessary reason to classify this non-baryonic mass as 'dark'.

As parejkoj noted above, CMB observations interpreted through cosmological models give consistent values for the mass-density of the observable universe, and its baryonic density as well (a key extra input is an estimate of the total number of photons in the observable universe). These relate directly to 'non-baryonic mass/matter' and by direct implication 'non-baryonic dark matter'.

Finally, never say "none". It is very important to recognise that summaries like this may leave the impression that there is no 'hot dark matter', or no 'cold baryonic dark matter', or ... Remember that we are focussed on the the first significant figure, not a complete balance sheet. So, for example, there most certainly is 'hot, non-baryonic dark matter' (neutrinos), 'hot, baryonic dark matter' (cosmic rays), 'cold {baryonic status indeterminate/undefined} dark matter' (the SMBH of normal galaxies), etc, etc, etc. However, these contribute to the observable universe's estimated mass down in the 3rd, 4th, or beyond significant place ...

Oh, and a note to experts: I am well aware that this post contains some simplifications, and you may well consider some of them to be so extreme as to be almost misleading. If you do so feel, please wade in with whatever caveats, clarifications, etc you feel necessary.

If anyone else would like to unpack the 'cold' and 'collisionless' attributes ...

* since then independently verified a great many times, in many different wavebands. IIRC, the Sun's bending of light from distant sources can be detected across most of the sky (VLBI observations of quasars); Jupiter's bending has been observed; and it is a 'confounding factor' that the GAIA data pipelines will have to account for, not only from the Sun and Jupiter, but also from all planets (except Mercury, which will always be too close to the Sun).

^ note that the other logical possibility - that the estimated baryonic mass is greater than the estimated total mass - has not (AFAIK) ever been encountered