Seven Misconceptions About Dark Matter

Dark matter seems able to attract the crazies like few other areas in astronomy. That's understandable, really. Sensible, even. Anyone half-rational ought to be a little bit suspicious about ivory-tower loons claiming that most of the Universe is made up of an invisible, massive substance lurking in outer space that makes galaxies spin too fast.

Original source.

But skepticism is one thing, denial is another. Many unqualified people insist that dark matter is somehow "obviously wrong", and sometimes they even get quite angry about it. They seem to think that astronomy is some kind of soft science that is somehow not based in the same understanding of physics that's given us things like, oh, I don't know... rockets. The internet. Nuclear weapons. Electricity. GPS satellites. Things that by and large work.


1) Dark matter is driving the universe apart.

It's pretty common to confuse dark matter and dark energy. In fact, they're completely different things. Dark energy refers to the observation that the expansion of the Universe appears to be accelerating. It's a catch-all term for various explanations as to why this is happening; no-one really know what's going on here.

Image credit : me.

Dark matter is a little less mysterious, but only a little. Observations of galaxies have revealed that they're rotating too fast and should all quickly fly apart according to accepted theories of gravity. Rather than driving things apart, like dark energy, dark matter is pulling them together. Astronomers like using the term "dark" as much as possible because a) it's super sexy and b) we're scared of daylight.

If dark matter disappeared...

2) Dark matter was discovered because of the movement of the stars.

This one is extremely common even in popular science articles. It's half-true. Jan Oort did postulate the existence of unseen matter back in 1932 based on observations of stars, but that evidence turned out to be pretty poor. Around the same time, the notoriously prickly Fritz Zwicky also decided that entire clusters of galaxies needed dark matter to hold them together, but he was such a complete jerk that no-one really cared*. A certain Horace Welcome Babcock (truly a wonderful name) did find evidence for dark matter in 1939 by looking at stars, but his evidence wasn't all that convincing either.

* Seriously, read the link. His colleague Walter Baade thought he was planning to murder him.

From left to right : Oort, Zwicky and Babcock.

The first really compelling evidence came in the 1970's when Vera Rubin found really good evidence that the gas in the outermost parts of galaxies was moving much too quickly - in fact it appeared to be moving just as quickly as the gas in the inner regions. Classical theories said it should be moving several times more slowly. But even though Rubin herself had uncovered evidence from stars that the rotation of the Milky Way remains flat, it wasn't until she (and many others) found the same evidence (by looking at the gas) in many tens and hundreds of galaxies, that people really started to become convinced. The astronomical community's initial skepticism was overcome only when the evidence was strong.

From wikipedia. The rotation speed of the gas stays high much
further from the centres of galaxies than predicted.

Gas in galaxies is usually more extended than the stars. Without dark matter, models predict that most of the time the stars at the edge of the disc should be just at the point where we should see their velocities start to drop. Gas makes it a lot easier to determine what's going on further away from the galaxy's center.


3) Dark matter is a fudge to make the data fit a model.

Yes, it is. But more accurately it's a fudge to make the data fit several models and many independent observations. The same dark matter postulated to explain galaxy rotation can also explain their motions in clusters (which should also quickly fly apart without dark matter), the large-scale structure of the Universe, gravitational lensing, and cosmological models predicting how much ordinary (i.e. visible) matter there should be (see point 4).

Simulation of the large-scale structure of the Universe, which
arises from the collapse of dark matter. Compare with observations
here.

Not so long ago, it would have been entirely reasonable to claim that dark mater is an invention designed solely to explain flat rotation curves in galaxies. Some other mechanism to spin-up the gas instead of the stars might have been invoked, but that's no longer the case. When you have multiple lines of inquiry pointing to the same result, it's not a fudge, it's a conclusion. It might not be the correct conclusion (and yes, we'll get to evidence against dark matter soon enough), but it's a perfectly legitimate interpretation of the data.

I've said it before and I'll say it again : believing what the evidence tells you is not somehow arrogant or dogmatic. It certainly isn't a fudge. It's science. Dismissing the evidence because you don't like the conclusion is arrogant and dogmatic - and in many cases, it most certainly is a fudge. Especially when that conclusion was determined by skeptical inquiry.


4) Dark matter could just be ordinary matter.

Obviously it can't be just normal stars or we'd see them. But perhaps it's just undetectable cold gas, or small compact objects like asteroids, dead stars or packing foam ? Small objects turn out not to be the case : observational limits (by looking for distortions in the light they'd produce in nearby stars) say that they can't account for more than a few percent of the missing mass.

Sorry Douglas.

Undetectable gas is much more intriguing. The idea pops up every so often* - and it's very tempting. Atomic hydrogen is relatively easy to detect - cold molecular hydrogen is nigh-on impossible. Instead, we look for another molecule (CO) which we think traces molecular hydrogen. But the relationship between CO and H2 is complicated, and thought to vary depending on the environment of the gas. So it's not at all beyond the bounds of possibility that we've got this wrong.

* This is quite readable (to the non-specialist) article by my PhD supervisor. It's not peer-reviewed I don't think he intended it as anything more than speculation (he was, after all, one of the co-discovers of VIRGOHI21).

There are of course some problems with this. Massive amounts of extra gas would mean that our entire theory of star formation is just plain wrong, which is a little hard to swallow. And - if you'll forgive me for thinking on the keyboard - we ought to see this gas when it gets shock-heated by dwarf galaxies passing through it.

These problems might not be fatal, however. The Big Bang theory predicts more ordinary matter than we see - just enough, it turns out, to explain the flat rotation curves of galaxies if it was distributed correctly. But not enough to explain the formation of the large-scale structures given the age of the Universe, or (I think) the motions of galaxy clusters, or fluctuations in the Cosmic Microwave Backgound. Now that could just mean the whole Big Bang model is wrong, but it's also very difficult to see how ordinary matter could explain some specific cases like the Bullet Cluster.

Overlaid on a normal visible light image, hot gas (from X-ray observations) is shown in red. Dark matter (inferred in this case from gravitational lensing) is shown in blue.

Here two galaxy clusters have collided. Since the gas is distributed over a very wide area, the gas in the two clusters collides and gets stuck in the middle. The stars (and dark matter) keep going. If the dark matter was some form of gas, it should have got stuck in the middle along with the rest of the gas. But it didn't. Gravitational lensing measurements indicate that it carried on moving with the stars, exactly as conventional wisdom predicts.

EDIT : More accurately, gravitational lensing measurements are consistent with the predictions of dark matter. However, the amount of lensing predicted in alternative theories of gravity (more on them soon) doesn't necessarily depend on the sheer mass that's present but on the distribution and geometry of that mass. So, advocates tell me, that all the lensing follows the stars and not the gas is not necessarily inconsistent with the fact that most of the mass is in the gas. However, I haven't been able to find a paper to confirm if this is the case.

The Bullet Cluster may not be the silver bullet against other interpretations of dark matter that many people would like (we'll get back to those later), but it's tough to see how ordinary matter could explain this with conventional theories of gravity.

Dark matter, then, is very difficult to explain as ordinary matter. Normally, "dark matter" is taken to mean an as-yet unknown type of particle which doesn't interact with "normal" matter except through gravity. The consistency between observations of galaxies, galaxy clusters, and Big Bang cosmology is an intoxicating success of modern science. But before we get totally wasted on scientific glory, we should beware the hangover that comes when we discover problems with the models. I'll return to those a bit later.



5) Inventing huge amounts of an unknown substance is a little bit bonkers.

Sure, but what's more outrageous : postulating an unknown substance, saying that our theory of gravity (which is tested to an incredibly high level of precision) must be wrong, or saying that some other utterly mysterious process is at work ?

Now it's true, we know relativity cannot be the be-all and end-all description of gravity. Singularities, points of infinite density where models become nonsensical, indicate that we're missing something. But dark matter is generally only important on much larger scales, where there is as yet no reason to suppose relativity doesn't work (just as Newtonian gravity, even though we know it's wrong, is still an excellent approximation in most situations).

This overlaps with the idea that it's a "fudge". Well, learning new things is the whole point of the scientific process. In the late 19th century, it was thought that there might be a planet closer to the Sun than Mercury, which would explain some slight anomalies in Mercury's orbit. That turned out not to be the case - Newton's theory of gravity gave way to Einstein's - which allowed a raft of new possibilities, not least of which is time travel. Vulcan was disproved, and most people today know it only as the even more fictitious homeworld of Mr Spock.

Skeptical Spock is skeptical of your skepticism.

In the case of Vulcan, scientists proposed missing mass to explain their theories. On that occasion, it didn't work - scientists were forced to the truly startling conclusions of relativity.  Dark matter, however, is proving a lot harder to dislodge. It looks like this time the theories are correct, but it's not for want of trying to come up with a better theory (see below). Here we're forced to the startling conclusion that most of the Universe is made of completely different matter to what we see around us.

Philosophically, I can't see any reason you'd prefer modifying an established, well-tested theory over the idea that there's an unknown substance - there's simply no way to avoid making a "mad" choice here. Pragmatically, the existence of neutrinos - which are similar (but not identical) to the postulated dark matter particles - indicates that dark matter is not such a silly suggestion at all. Maybe dark matter doesn't exist, but I find it hard to see the alternatives as being any less radical.


6) But surely, if it disagrees with experiment, it is wrong ! Galaxy rotation curves are flat, therefore relativity is wrong and we need a better theory.

Partial credit ! It's extremely important to remember - and not stated nearly often enough - that dark matter is theoretical*. We don't know for certain that it exists, but you'd be forgiven for thinking from some articles that astronomers believe in dark matter in the same way that geologists believe in rocks. This isn't the case at all. A few do. Most simply accept its reality for convenience : it becomes impractical to give a long-winded explanation for dark matter's existence in every press release.

* Yes, you can say "only a theory", provided you really understand what this means.

It's entirely possible that if galaxy rotation curves had been found to be flat much earlier, when relativity was not firmly established, this might have caused severe problems. Instead, relativity was tested time and time again and found to be successful. That hasn't stopped some people from considering that maybe the flat rotation curves are the ugly factual flies in the ointment that may yet slay the beautiful Goliath theory that is relativity and/or modern cosmology.

And there are other anomalies with dark matter models. The number of dwarf galaxies predicted in simulations is much higher than what's observed in reality (some of these missing galaxies are worryingly large). Their distribution is all wrong. Dark matter particles weren't predicted by the Standard Model. There are lots of things in cosmology and galaxy evolution we don't understand, and anyone who says otherwise is quite, quite mad*.

* Many of these problems might be solved by better understanding of the physics of the ordinary matter involved. They do not automatically necessitate rejecting dark matter. "If it disagrees with experiment it is wrong" is a laudable principle, but you have to be sure it disagrees.

Planes of satellite galaxies (source). In normal cosmology smaller galaxies should be found in spherical distributions around their host galaxies. In reality this is not true, at least for the Milky Way and Andromeda (M31).

EDIT : Since writing this, a paper has been submitted which shows that such planes of satellites can form in standard dark matter simulations. In this case such planes are present around 15% of galaxies which formed particularly early (which the paper claims is the case for the Milky Way and Andromeda), possibly higher if those galaxies don't subsequently experience a major merger. At present, the paper does not give enough information to really say if planes around galaxies like ours are really unusual or quite common, but it does demonstrate that they can form.

The leading alternative idea to dark matter is MOND : MOdified Newtonian Dynamics (though I've always preferred the name MONG : MOdified Newtonian Gravity). This, as the name implies, avoids dark matter by suggesting a different theory of gravity, which is very similar to Newtonian gravity except when accelerations are very low.

MOND isn't pseudoscience - it offers some genuinely very interesting insights. For example, faint galaxies are known to follow similar scaling relations to bright galaxies, which conventional theories say they shouldn't. MOND explains this quite naturally. The distribution of dwarf galaxies is explained in MOND by most of them being formed in interactions between larger galaxies - that's why they're found in narrow planes around larger galaxies, rather than the spherical clouds predicted by standard models.

But MOND has its own problems. It's not so straightforward to change simulations to use MOND's law of gravity. This means we don't yet know whether a MONDian simulation of the Universe would do as good a job as dark matter does. And MOND does require some dark matter, though so little it could plausibly be ordinary matter rather than the more exotic kind of orthodox cosmology. It's also damn hard to find a theory of MOND that fits some of the known effects of general relativity.

The latest challenge on that front is the orbital decay of a double pulsar. GR describes this brilliantly, MOND fails. It appears that it may be difficult for MOND to simultaneously model both the Solar System, where gravity is weak, and pulsar systems, where it is strong - GR doesn't have this problem.

Double pulsar. GR has successfully predicted the orbital behaviour, MOND, allegedly, has not.

Yet pronouncing MOND as dead because of this is surely premature. I have not been able to find any response from MOND supporters regarding this. Until they do, MOND might be regarded as wounded  - perhaps fatally, perhaps not - but still very much alive; perhaps writing its last will and testament or perhaps rallying for a fresh assault. Pronouncing it dead now is like burying a man without checking if he's breathing : it runs of the risk of creating a terrible zombie-vampire MOND that will wreak havoc upon us all.

RAAAR ! I AM ZOMOND ! FEAR ME, FOR I SHALL
 CRUSH YOUR THEORIES WITH WELL-REASONED
 PHILOSOPHICAL ARGUMENTS !

EDIT : My MOND buddies tell me that indeed the best-studied version of MOND that's compatible with relativity is indeed disproved by this. However that doesn't rule out that there may be some other theory of gravity that could explain both solar system and pulsar measurements. In my view, that still means you're left with an uncomfortable choice : hoping that a new theory will come along and explain everything, or hoping that a dark matter particle will be detected.


7) Well, pish. Scientists dogmatically insist that dark matter must exist without any real evidence, because they haven't got any better ideas. So there.

This one is wrong every which way you look at it. For starters, there are alternative theories, but they have difficulties. They have yet to match the stunning self-consistency of modern cosmology, which successfully describes how quantum fluctuations can give rise to the galaxies and galaxy clusters we see today. Sure, there a ton of problems of detail. And it's been heavily modified along the way - maybe the day will come when we will have to declare it broken. But it is not this day.

The incredibly wise Vera Rubin said is thusly :

"One hundred years ago, galaxies were an enigma. They still are. It is folly to believe that we know what a galaxy is, while the extent, the density distribution, and the composition of more than 90% of its mass are still a dark mystery...We have learned much about galaxies in the last 100 years. I think that we still have major surprises to uncover."

It's worth bearing in mind just how young modern astronomy is, and the scale of the challenges it faces. 80 years ago we didn't know that other galaxies were distant objects. 40 years ago we had no idea that dark matter might exist, except for a few vague hints. When inferring the existence of a mysterious substance that's more prevalent than "ordinary" matter, is it really so surprising that we don't understand everything ?

Galaxies like this one are incredibly complex systems. Hundreds of billions of stars would be enough, but they also contain a roughly equal mass of gas and varying amounts of dust. Nor do they exist in isolation - they're surrounded by many smaller satellite galaxies, and sometimes hundreds of giant companions. So when a world view is as successful as explaining them as modern cosmology, it makes no sense to declare it invalid as soon as the slightest problem is found.

Rubin again :

"In a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That's probably a good number for the ratio of our ignorance-to-knowledge. We're out of kindergarten, but only in about third grade."

Moreover, experiments are underway to test the reality once and for all, to put it on the same level of certainty that geologists claim for rocks. Scientists are indeed putting their money where there theories are. The "dark matter is stupid" ilk might have a point if such experiments were not taking place.

I said earlier that it's not arrogant to believe the evidence. For most people, the evidence favours dark matter. For some it favours MOND. When there's evidence either way, it's a judgement call as to which side you want to pick. MONDers aren't necessarily arrogant, but those who unjustly claim that dark matter must/must not be real certainly are.

Even if MOND is falsified, it doesn't prove dark matter exists. Only a direct detection would constitute true certainty. But for me, and most astronomers, the spectacular self-consistency of dark matter cosmology makes it more probable that it does exist. Maybe I'm wrong. One day there might be a revolutionary discovery that overturns the whole notion of dark matter - if so, astronomers everywhere will not drown their sorrows : they will celebrate. Until that happens, dark matter cosmology is the best we've got.