# Has dark matter been ruled out? (March 2018 news)

I watched a YouTube post on a channel that normally has solid information about astronomy (as far as I can tell). But this episode states that (emphasis mine):

…talking about the controversial dark matter…

…its most-likely non existence.

We think (or we thought — that is, past tense) that there was a lot of dark matter out there in the universe…

[@3:38]: …they just recently published their final results and just like the ones in China just like the ones in Europe they were basically inconclusive.

What does it mean? Well it means that they found nothing. They found that there is no particle called dark matter; they found that our theories of what dark matter consists of is not really correct and the best theory which resolved around something called wimps … was inconclusive. We found nothing; we literally found zero. We found absolutely zilch not even close to finding a little bit.

Personally, I think he misunderstood ruling out of specific parameters for ruling out DM in general. But I ask of those who are knowledgeable about the subject, have recent results ruled out dark matter from existing? If not, what’s the real story here: have Xenon detectors concluded that some specific mass and interaction profile been ruled out, or what?

P.S.
On a site that normally has good information, it’s important to point out the occasional mistake or goof. I ask that someone who knows the subject also post a comment on the video. And, readers here who can attest to the credibility of the comments critical to the video please upvote those comments.

• I wouldn't trust a generic Youtube channel for anything, really. I don't know of a single one that is reliably correct. – knzhou Mar 20 '18 at 12:01
• The many problems that dark matter is thought to solve are still there, same as ever, but experiments are slowly ruling out one particular model of dark matter (WIMPs). I think that's been known for at least a decade, and generic WIMPs are just one of many possible types of dark matter. – knzhou Mar 20 '18 at 12:02
• @knzhou PBS Spacetime is a pretty solid channel, IMO. And Physics Girl and FermiLab. – StephenG Mar 20 '18 at 12:18
• "They found that our theories of what dark matter consists of is not really correct" Is a sign that the writer (a) is not in the dark matter community and (b) has a unsophisticated understanding of how real scientific investigations work in the modern era. The hasn't been well defined "theories of what dark matter consists of ", there are a host of plausible models, and there are a couple of models that are experimentally tractable. Recently the focus has been on WIMPs because we stand a chance of (definitively-failing-to) detect them. Not because it was "the theory". ... – dmckee Mar 20 '18 at 19:57
• And while the WIMP data is starting to impinge upon the range where detection might happen, it certainly doesn't constitute a definitive ruling out as yet. And axions and other more exotic options are waiting in the wings. – dmckee Mar 20 '18 at 19:59

The problem in the quotes you give is a confusion between the need for the existence of dark matter, and the possibilities of what type of particles this dark matter is composed out of.

WIMPS are one possibility which depends on weakly interacting massive particles , which are actively searched for in the LHC experiments, but not found (yet?). They seem to decide that they do not exist, which is premature in my opinion, as we do not have a unified theory of the four forces at the moment.

There are also the MACHOs , massive compact halo objects, which also could be the dark matter objects.

A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter that emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs are not luminous, they are hard to detect. MACHOs include black holes or neutron stars as well as brown dwarfs and unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs. The term was coined by astrophysicist Kim Griest.1

The observations for the need of dark matter are there, unchanged afaik, so the video is at fault, if you are quoting correctly.

• Anna, should not the machos have been the first eligible to explain DM? In a sense their are an extended version of historical astronomical dark matter , such as meteorites dust brown dwarf .... Why to my little knowledge the hypothesis that DM is invisible ordinary barionic natter has relatively little momentum? Is it just because experts found difficult to motivate the existence of so much non luminous body and gases? I hope my question is clear. If so you might suggest me to open a new Q – Alchimista Mar 20 '18 at 14:22
• @Alchimista Both Machos and Wimps are speculations. Wimps are more normal in cosmological histories as the Big Bang, they would have to be long lived enough and weakly interactive left overs from the separation of the quantum mechanical phase into neutral matter. Machos we do not see in our solar system ( they should be five times the visible matter, i.e. the sun, in our neighborhood) . So elaborate models have to be built of black holes and dwarfs not radiating etc, i.e. concentrated invisible masses, at galactic scales. which is being attempted. – anna v Mar 20 '18 at 14:35
• Don't forget the WISPs/Axions as a possibility for cold dark matter on the other end of the mass scale! – Densch Mar 20 '18 at 15:24
• @annav, it is apparent to me that some of the theoretical physicists have a sense of humor. We are now looking for WIMPS and MACHOS! – David White Jul 30 '18 at 1:29

Dark matter has not been ruled out. To the contrary, it's been almost entirely ruled in.

What is meant by that is that there is evidence that not only are there anomalous gravitational effects that dark matter can explain, among a host of other possibilities, but that these effects specifically behave as though they are being produced by an actual, independent substance, and not, say, a modification of the laws of physics which would be expected to apply everywhere.

In particular, to understand this, we have to look at the very first evidence that was used to suggest the existence of dark matter: the famous "galaxy rotation curves", which are essentially a plot of the average speed at which a star in a galaxy is moving around its centre compared with its distance from the centre. As one may know from elementary physics, the orbital speed of a body about a single gravitating central object is proportional to $d^{-1/2}$, where $d$ is the distance from the gravitator, so it drops asymptotically to zero as the distance goes up. In a real galaxy, the curve drops more slowly, because instead of all the gravity coming from a point source, it comes from the other stars which are more spread out and come up to and around the star in question, allowing it to receive a more uniform amount of force compared to distance. The more sources of gravity there are and the larger, the more slowly the curve will drop off. And what is seen is that the curves fall off too slowly, even when you sum up all the gravity of the other stars, gas, and dust that make up the galaxy.

So of course a natural way out of the problem is to try and posit there's some additional matter in place which slows the falloff in the same way: more gravitating mass to keep the force strong and speeds thus high. The trouble is, you don't see any such mass, so you posit for some reason - to be determined - that it must not interact much or at all with the usual tool you use to do observations: here, electromagnetic radiation.

Of course, that also feels kind of like a cheat: like pulling an invisible pony out of a hat to get you out of a tough situation. And it also may, to someone with a bit of a historically minded eye, remind one of the controversy in the 19th century surrounding discrepancies in the orbital dynamics of the planets of the solar system, most prominently seen in Mercury, which were to at first be explained by positing the presence of unseen planets to provide the extra force and they did not turn up, and instead it turned out that these discrepancies were revealing something far more profound: that Newtonian gravity was wrong, and that a new replacement was needed which came in the form of Einstein's general theory of relativity. And likewise, given that experience, we might also wonder if something analogous is going on here and if maybe the apparent rotation curve discrepancy is actually due to something like that gravity inherently falls off more slowly than the inverse square law, i.e. instead of $F_G \propto \frac{1}{r^2}$ we have $F_G \propto \frac{1}{r^{2 - \epsilon}}$, for some $\epsilon > 0$, or some other suitably slowly-decreasing function of distance, and this modifying parameter is due to some new kind of physics of the same type - an exciting possibility that many are desperate for given theoretical and conceptual problems with our existing and best theories that seem to so far be, ironically, stubbornly good - too good, almost. Especially regarding the reconciliation of gravitational theory with quantum mechanics: general relativity and quantum mechanics famously don't like each other. If something new can be found regarding gravity's behavior that would be inconsistent with general relativity, it might require it to be modified or replaced by something else that is actually consistent. And so it's natural to posit that's what we're seeing.

The trouble is - we've recently now seen something else: in particular, there was an observation of a galaxy called NGC1052-DF2 - a very thin, wispy galaxy, a so-called "dwarf galaxy": a low mass galaxy of small size and few stars. There are a number of these around, but this one is particularly low-density and, moreover ... when it was looked at more closely, something very strange was observed:

The galaxy's rotation curve was 'normal'.

Meaning, it looked the same way that astronomers thought all along a galaxy's rotation curve was supposed to look, under the assumptions purely of normal gravity and the only contributors to that gravity being stars, stellar remnants/corpses, gas, and dust. It fell off with distance at the correct rate.

And this is a death blow to virtually any plausible alternative theory of dark matter that posits some form of modification to the laws of physics since, as far as we can tell, there is no reason the laws of physics should hold in some galaxies and not others, much less only the law of gravity. Rather, what it shows is that dark matter is indeed a substance, because it can be absent from a region, and thus not exert its influence.

And this is not the only object of its kind. Another was the Bullet Cluster, a set of colliding star clusters for which the gravity distribution had been probed by gravitational lensing and showed hints that the dark matter was also moving independently (thus confirming it as a substance in the same fashion), but there were still some modified gravity theories that might have been allowed by this. However, combined with this latest observation, I think it's pretty safe to say the "dark matter is a substance" conclusion is rather strongly ruled in.

The question then really shifts to what dark matter is - that is, what kind of substance it is, and that's where it gets frustrating. The most common types of searches for it are based on the assumption it's some new kind of elementary particles that are not part of the usual Standard Model of particle physics, which surround and fill up galaxies in the manner of a gas. The particles are posited to have low self-interaction and interaction with ordinary matter including and especially the electromagnetic force and thus light (this is relatively simple - just take the particles as charge neutral, but there's the possibility that they may interact by other interactions like the weak force, and thats' a bit harder to diminish: while that would make it low-interactivity enough, the weak force interaction for the only other particle we know that is like this and part of the Standard Model, the neutrino, is nonetheless interactive enough to permit its detection in very large, specialized detection systems, and it's also known neutrinos are not copious enough nor are they the right energy profile to account for most of the dark matter.).

Yet of course, we would naturally expect there to be at least some interaction, however slight, and many attempted theories to try and explain them suggest there should be these slight interactions. Moreover, the particles have to be suitably heavy or massive so that they are slow enough as to not escape the galaxies they are held to (i.e. to not exceed the galactic escape velocity.), given that without interaction they also have no easy way to radiate and give off energy. This then naturally leads to positing the idea of "WIMPs" - or "weakly interacting, massive particles". And many searches have been made using various kinds of hyper-sensitive detectors to try and pick these up, but they have so far given nothing. Of course one can always suppose the interaction is even less, but then we have to wonder where we start to draw the line between reasonable explanation and ad hoc special pleading.

Another possibility is that dark matter is composed of more ordinary objects, or at least known ones, like a swarm of black holes orbiting the galaxy. This is called the "MACHOs" - "Massive, compact halo objects" proposal. The difficulty with this proposal is black holes are known to gravitationally lens if they pass in front of a star, and such lensing has not been observed. Likewise as in the WIMP proposal, we can assume the masses of the black holes or other objects are smaller, and thus explain this ... up to a point: if the black holes' masses are too small, they would have evaporated by now by Hawking radiation, and if it's something else, there may be no plausible way to form such a small-mass clump without anything else likes tars, etc. forming.

So that's where we're at. Every bit of new evidence that comes in makes the conception of dark matter as a substance, as opposed to an artifact of something superficially considered "deeper", become stronger and stronger, yet also seems to render more and more enigmatic the nature of that substance. And that itself might be able to reveal deep, new physics that is potentially every bit as interesting as that that a modification of gravitational laws might reveal.

• Regarding the dwarf galaxy, see section 3.1 of arxiv.org/abs/1804.04167 .. There is velocity data for 10 objects in the galaxy. When all are included, the data is consistent with the MOND version of modified gravity. – Mitchell Porter Jul 30 '18 at 7:03

## protected by AccidentalFourierTransformJul 30 '18 at 15:37

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).