Dark Matter Exists

The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twenty-first-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Here’s the press release, and an article in the Chandra Chronicles.)

A prerequisite to understanding the dark sector is to make sure we are on the right track. Can we be sure that we haven’t been fooled into believing in dark matter and dark energy? After all, we only infer their existence from detecting their gravitational fields; stronger-than-expected gravity in galaxies and clusters leads us to posit dark matter, while the acceleration of the universe (and the overall geometry of space) leads us to posit dark energy. Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einstein’s general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances?

A departure from general relativity on very large scales isn’t what one would expect on general principles. In most physical theories that we know and love, modifications are expected to arise on small scales (higher energies), while larger scales should behave themselves. But, we have to keep an open mind — in principle, it’s absolutely possible that gravity could be modified, and it’s worth taking seriously.

Furthermore, it would be really cool. Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. (And Vera Rubin agrees.) We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isn’t to express preferences, it’s to suggest hypotheses and then go out and test them.

The problem is, how do you test an idea as vague as “modifying general relativity”? You can imagine testing specific proposals for how gravity should be modified, like Milgrom’s MOND, but in more general terms we might worry that any observations could be explained by some modification of gravity.

But it’s not quite so bad — there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle. So if we imagine doing away with dark matter, we can safely predict that gravity always be pointing in the direction of the ordinary matter. That’s interesting but not immediately helpful, since it’s natural to expect that the ordinary matter and dark matter cluster in the same locations; even if there is dark matter, it’s no surprise to find the gravitational field pointing toward the visible matter as well.

What we really want is to take a big cluster of galaxies and simply sweep away all of the ordinary matter. Dark matter, by hypothesis, doesn’t interact directly with ordinary matter, so we can imagine moving the ordinary stuff while leaving the dark stuff behind. If we then check back and determine where the gravity is, it should be pointing either at the left-behind dark matter (if there is such a thing) or still at the ordinary matter (if not).

Happily, the universe has done exactly this for us. In the Bullet Cluster, more formally known as 1E 0657-56, we actually find two clusters of galaxies that have (relatively) recently passed right through each other. It turns out that the large majority (about 90%) of ordinary matter in a cluster is not in the galaxies themselves, but in hot X-ray emitting intergalactic gas. As the two clusters passed through each other, the hot gas in each smacked into the gas in the other, while the individual galaxies and the dark matter (presumed to be collisionless) passed right through. Here’s an mpeg animation of what we think happened. As hinted at in last week’s NASA media advisory, astrophysicists led by Doug Clowe (Arizona) and Maxim Markevitch (CfA) have now compared images of the gas obtained by the Chandra X-ray telescope to “maps” of the gravitational field deduced from weak lensing observations. Their short paper is astro-ph/0608407, and a longer one on lensing is astro-ph/0608408. And the answer is: there’s definitely dark matter there!

Despite the super-secret embargoed nature of this result, enough hints were given in the media advisory and elsewhere on the web that certain scientific sleuths were basically able to figure out what was going on. But they didn’t have access to the best part: pictures!

Here is 1E 0657-56 in all its glory, or at least some of it’s glory — this is the optical image, in which you can see the actual galaxies.

1e0657 optical

With some imagination it shouldn’t be too hard to make out the two separate concentrations of galaxies, a larger one on the left and a smaller one on the right. These are pretty clearly clusters, but you can take redshifts to verify that they’re all really at the same location in the universe, not just a random superposition of galaxies at very different distances. Even better, you can map out the gravitational fields of the clusters, using weak gravitational lensing. That is, you take very precise pictures of galaxies that are in the background of these clusters. The images of the background galaxies are gently distorted by the gravitational field of the clusters. The distortion is so gentle that you could never tell it was there if you only looked at one galaxy; but with more than a hundred galaxies, you begin to notice that the images are systematically aligned, characteristic of passing through a coherent gravitational lens. From these distortions it’s possible to work backwards and ask “what kind of mass concentration could have created such a gravitational lens?” Here’s the answer, superimposed on the optical image.

1e0657 optical and dark matter

It’s about what you would expect: the dark matter is concentrated in the same regions as the galaxies themselves. But we can separately make X-ray observations to map out the hot gas, which constitutes most of the ordinary (baryonic) matter in the cluster. Here’s what we see.

1e6057 optical and x-ray

This is why it’s the “Bullet” cluster — the bullet-shaped region on the right is a shock front. These two clusters have passed right through each other, creating an incredibly energetic collision between the gas in each of them. The fact that the “bullet” is so sharply defined indicates that the clusters are moving essentially perpendicular to our line of sight.

This collision has done exactly what we want — it’s swept out the ordinary matter from the clusters, displacing it with respect to the dark matter (and the galaxies, which act as collisionless particles for these purposes). You can see it directly by superimposing the weak-lensing map and the Chandra X-ray image.

1e6057 optical, dark matter, and x-ray

Clicking on each of these images leads to a higher-resolution version. If you have a tabbed browser, the real fun is opening each of the images in a separate tab and clicking back and forth. The gravitational field, as reconstructed from lensing observations, is not pointing toward the ordinary matter. That’s exactly what you’d expect if you believed in dark matter, but makes no sense from the perspective of modified gravity. If these pictures don’t convince you that dark matter exists, I don’t know what will.

So is this the long-anticipated (in certain circles) end of MOND? What need do we have for modified gravity if there clearly is dark matter? Truth is, it was already very difficult to explain the dynamics of clusters (as opposed to individual galaxies) in terms of MOND without invoking anything but ordinary matter. Even MOND partisans generally agree that some form of dark matter is necessary to account for cluster dynamics and cosmology. It’s certainly conceivable that we are faced with both modified gravity and dark matter. If the dark matter is sufficiently “warm,” it might fail to accumulate in galaxies, but still be important for clusters. Needless to say, the picture begins to become somewhat baroque and unattractive. But the point is not whether or not MOND remains interesting; after all, someone else might come up with a different theory of modified gravity tomorrow that can fit both galaxies and clusters. The point is that, independently of any specific model of modified gravity, we now know that there definitely is dark matter out there. It will always be possible that some sort of modification of gravity lurks just below our threshold of detection; but now we have established beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics.

That’s huge news for physicists. Theorists now know what to think about (particle-physics models of dark matter) and experimentalists know what to look for (direct and indirect detection of dark matter particles, production of dark matter candidates at accelerators). The dark matter isn’t just ordinary matter that’s not shining; limits from primordial nucleosynthesis and the cosmic microwave background imply a strict upper bound on the amount of ordinary matter, and it’s not nearly enough to account for all the matter we need. This new result doesn’t tell us which particle the new dark matter is, but it confirms that there is such a particle. We’re definitely making progress on the crucial project of understanding the inventory of the universe.

What about dark energy? The characteristic features of dark energy are that it is smooth (spread evenly throughout space) and persistent (evolving slowly, if at all, with time). In particular, dark energy doesn’t accumulate in dense regions such as galaxies or clusters — it’s the same everywhere. So these observations don’t tell us anything directly about the nature of the 70% of the universe that is purportedly in this ultra-exotic component. In fact we know rather less about dark energy than we do about dark matter, so we have more freedom to speculate. It’s still quite possible that the acceleration of the universe can be explained by modifying gravity rather than invoking a mysterious new dark component. One of our next tasks, then, is obviously to come up with experiments that might distinguish between dark energy and modified gravity — and some of us are doing our best. Stay tuned, as darkness gradually encroaches upon our universe, and Einstein continues to have the last laugh.

This entry was posted in Science. Bookmark the permalink.

197 Responses to Dark Matter Exists

  1. Kneemo says:

    Awesome pictures Sean!

    My favorite is the superimposed weak-lensing/Chandra X-ray image. I’m trying to understand the bullet-shaped shock front. Is the bullet-shape the result of ordinary matter being gravitationally “dragged” by the dark matter cluster?

  2. Markk says:

    If MOND were true, would this not imply that a smaller amount of matter (or dark matter) would be able to bend light more strongly than predicted by general relativity if it passed by at a greater distance to the matter causing the bending?

    Yes, that would be one prediction, but it is not really relevant to this result, since perhaps 90% of the ordinary mass is in the “pink” regions in the picture above, yet the lensing is happening in the blue regions which thus have only 10% of the mass. it does not matter if there were different lensing rules, assuming they were still symmetric under the centers of mass which seems likely. Any lensing caused by regular matter would be more in the pink regions. Since it isn’t there must be something in the blue regions, something we can’t see, causing the lensing…

  3. Arun says:

    Shouldn’t the dark matter distribution of each galaxy be tear-drop shaped or otherwise distorted because of the gravitational interaction with the other galaxy? Or is it a result that two spherical distributions of matter that pass through each other remain spherical?

  4. Luis Sanchez says:

    Hello Sean, Can I use some of the images of this post in my blog?

  5. Sean says:

    Luis, sure, just link to the NASA site linked above.

    Arun, if you look closely, the DM distribution is distorted. However, the precision of the technique isn’t sufficient to nail that down with a great deal of confidence, is my bet.

  6. Acolyte says:

    Acolyte, it’s not just the angle; it’s the direction in which the light is bent. The important thing here is that the gravitational field is pointing somewhere other than where the ordinary matter is located.

    Ok, I realize that the gravitational field doesn’t seem to be focused on where the visible matter is.

    I don’t know much about the mathematics of gravitation, so here is a different question: is the solution unique? That is, is the only explanation for the observed bending pattern the presence of M amount of dark matter in the location shown. Or, is this hypothesis one of a family of possible solutions to the problem.

    As an analogy, suppose someone throws a ball at me, straight to my left eye, but something deflects the ball so that it hits me on the right corner of my lip. So, the ball is deflected by a a certain amount in the bottom-right direction. But, this could have happened either because someone nudged the ball by a little amount shortly after it was launched, or by a greater amount as it was getting closer to me.

    So, the question is: how certain are we that the thing that did the nudging is right there close to the galaxies and not somewhere else? Is it perhaps that there is a unique solution to the bending of light from all the background galaxies, so that it could only be there?

    Another question: could the gravitational field be strong in the wrong place for a different reason, say, the presence of a black hole that has sucked in all the matter in its vicinity? Or can this be ruled out?

  7. Chaz says:

    But I don’t think it’s quite right to imply that Bekenstein’s theory could match the lensing observations. His theory is supposed to get the magnitude of the lensing right in a circularly-symmetric profile, but I don’t see how it could give rise to two big blobs of gravitational field offset from the mass distribution.

    Sean, have you seen this paper? Can you comment?

  8. B says:

    Hi Sean,

    The DM should clump on all scales, no reason why not. But we don’t have any direct evidence

    Okay, thats also my understanding of the situation. I was just puzzled by your statement that it does clump even into star-sized things.

    Thanks for the clarification.

    Best, B.

  9. Sean,

    Great post, pics, links and animations.

    One hopelessly anthropomophic question: About the X-rays from that 160 million degree plasma – would they fry any life on the trillions or so planets in the galaxies involved in the collision, or is the intensity too low?

  10. JDub says:

    Over the last several days, I created a video from a physically motivated simulation of the Bullet Cluster. Last week Maruša asked me if I could make an animation, and I agreed. I thought it would be best to run an actual simulation rather than guessing at the collision’s dynamics. I used Enzo for the calculation. This was used in the Stanford/SLAC press release. Take a look…

    Bullet Cluster Visualization

  11. Paul says:

    Since you guys ignore the EM force which is 10^39 greater than G I think you guys should wake up! This animation shows exactly what you would expect from an EM Field interaction and the X-Rays would be the natural conclusion. This isn’t Proof of Dark Energy or Dark Matter. It is just the illogical conclusions of people who keep refusing to see the obvious electromagnetic interactions of the Universe. It qualifies more as Garbage In and Garbage Out of a computer simulation than the realization that standard Maxwell interactions predict this and nothing else really does. Of course Red Shifts will not be velocity either. They are simple optical tuning effects in space. That will crank out all of the nonsense of a failing theory.

  12. Andrew says:

    Paul: right on! When will these people realize that there are four simultaneous 24-hour days in a single rotation of the earth? Creation occurs via opposites. Singularity is the death math of religious/academic Godism. Earth Opposites should split apart – and cascade molten lava upon God Worshippers, for they are the evil on Earth.

  13. Pingback: Clear Ambiguity

  14. Henry Holland says:

    [proud parent voice] Our son finally made it! He got quoted on the BBC website! [wipes tears away]

    Fantastic post that made sense to this layman. Great news; it really is an exciting time for your field, isn’t it?

  15. Raph says:


    Informative (and entertaining) as always. You know I still pull out those GR notes of yours from time to time.


    You’re right about the intensity. The IntraCluster Medium is an interesting assembly of matter, because it is so low in density, perhaps one lonely particle per 1000 cubic centimeters, if my memory from grad school is right. It is hot, in large part, because it can not cool itself effectively because interactions between the widely spaced particles are rare. The typical cooling time is longer than the age of the universe, except near the centers of massive clusters. So the actual rate at which energy is radiated away (or at planets) is low.

  16. Pingback: born live love die » Dark Matter matters

  17. SFOtter says:

    70% of the universe is ultraexotic?

    Isn’t that an oxymoron?

    Add why do I live in the boring 30%?

  18. Pingback: What You’re Doing Is Rather Desperate » Dark matter and hobbits

  19. Hyperion says:

    I’m not a physicist (stopped taking physics once I got to college and realized that I was calculus-impaired), so this is probably a rather ignorant question, as it is likely either physically impossible or so obvious that it’s already been considered:

    If it is known to be non-baryonic, is it possible for dark matter to consist of some sort of large clump of massive bosons functioning in some unknown manner?

    Or would it be possible for supermassive black holes at the center of these galaxies to pass by or through each other in some manner, or maybe even slingshot around each other, with the “matter” of the galaxies either being ejected or being slowed down by various collisions and interactions, and what we’re seeing are two semi-naked black holes (which cause the gravitational lensing and would comprise the majority of the mass), flying away from each other, eventually pulling apart the cloud of matter from the collision behind them into the two red gas clumps we see in the picture?

    Granted it wouldn’t explain any of the other measurements showing that that there couldn’t have been enough matter in the universe to account for its effects. Like I said, probably very ignorant, but when I think of something large and massive and invisible, that would have to come to mind.

  20. Eugene says:

    Hi Scott and Sean,

    Well, a cursory reading on the Angus et. al. (2006) paper cited by the Letter is rather interesting. In that paper, they show that you can actually construct multi-center mass distributions, with the lensing maps not projected onto the centers, in the TeVes framework. This has something to do with the complicated direct vector field interaction with metric.

    In Angus et. al., they claim that for certain classes of models of TeVeS, Sean is indeed right that there exist still a baryon discrepancy (i.e. they can’t get enough lensing). However, they only consider some classes of models, and they have not fully explored all the parameter space of TeVeS (e.g. the case where the Chi field is important).

    The Letter mentioned two problems with such a construction : that the ‘multicenter’ maps that Angus et. al. constructed has all the centers lined up, and that for the Angus model to work there must exist a small central mass concentration (between the two subclusters). Neither is true observationally : the small central peak is consistent with standard Newtonian prediction and the peaks do not line up.

    The bottom line is : TeVeS does not necessarily predict a lensing profile that scales simply (this will be ruled out by the many observations of colliding clusters, including this one). Indeed, Angus et. al.’s paper was to demonstrate this fact, making the point that TeVeS does not have to reduce to MOND. However, there exist plenty of parameter space to be explored. TeVeS is quite, to put it mildly, messy.

    MOND probably dead as a empirical law. TeVeS, on the other hand, may still be alive but is getting as baroque as the multiparameters that we need to cook up for DM to make it all fit.

  21. Eugene says:

    Actually, after hitting “submit”, I think I take back my statement about MOND is dead as an empirical law. MOND is very much alive as a empirical law that “predicts” the rotation curves. But it does not say anything about lensing. Any relativistic extenstion to MOND that predicts a simple scaling of lensing with baryonic mass distiribution is dead, but no such theory exist. (TeVeS does not predict a simple scaling in multicenter models.)

  22. Digital says:

    I see over and over stated that “Dark matter clumps to galaxys and clusters”… but here’s a turn of thought.

    Why not have it be that matter tends to clump to regions where dark matter has clumped. Maybe it’s all chicken-and-egg thinking, but why lock ourselfs into thinking that the dark matter is running around looking for normal matter to hang out with? Bottom line, maybe normal matter is following it around.

    Regardless though, great reading. My hat is off to the people that make expanding our knowlage on the extream edge possible.

  23. Pavlita says:

    Why has no scientist tried to prove the Einstein-aether theory, because in that theory free energy is possible. And capitalism wont allow free energy to be sold, because there isn’t enough money in that.

    On a side note, dark matter is in pop culture alot, and we want to be able to keep using the word “Dark Matter” so we try to prove it exists, because it sound cool.

  24. JoAnne says:

    Great post Sean! I am very relieved to be rid of MOND for good and for all.

    I am a bit confused about the process of releasing the results….NASA had the big press conference today, and the papers are just now appearing. Yet (as I was reminded today) Michael Peskin showed the pictures in his lecture at the SLAC Summer Institute. Obviously he had “hot” copies because part of the analysis was done here at SLAC at the Kavli Institute.

    Gosh, this brings home my biggest fear of how the release of data is going to be handled at the LHC…

    PS: I tried hard to link to Michael’s SSI talk,

  25. Pingback: My Bag of Beans » Blog Archive » NASA Finds Direct Proof of Dark Matter