Oct 05 2009

Is Dark Matter Real?

Dark matter is cool – not necessarily its actual temperature, but as a scientific concept. It is one of the scientific mysteries of our generation, and it’s a fascinating story that is unfolding before our eyes with each new discovery. It’s just a good science story. A recent observation has just added to the drama, perhaps calling into question dark matter’s existence.

Dark matter is a helpful example of how the scientific process sometimes works. Scientists first proposed the notion of dark matter to explain anomalies in the rotation of galaxies – they rotate faster than they should, indicating that they have more mass than is visible. It’s as if they have lots of matter that is not visible – dark matter.

Dark matter became a working hypothesis to explain our observations about the universe, especially how galaxies move. But all scientific theories, in order to be of any value, must not only have explanatory power, they also need predictive power – they must make predictions that allow the theory to be tested.

One piece of evidence that is thought to be confirmation of dark matter is the bullet cluster. This is an area of space where two galaxy clusters are colliding. Astronomers can see that the gas clouds slowed down as they crashed into each other. But the stars mostly passed right by each other. It also appears that the dark matter in the clusters also just went past each other, as the gravitational lensing of the areas where there are lots of stars is much greater than the stars alone can account for.

However, there is an alternate theory to dark matter – called MOND for modified Newtonian dynamics. Since dark matter only appears to interact with other matter through gravity (like in the bullet cluster, it was not physically slowed down by the collision – only its gravity is evidence), then perhaps, says MOND, all we need to do is tweak gravitational theory to account for the apparent effects of dark matter.

Specifically, if gravity does not fall off as quickly as we currently think then we could explain the rotation of galaxies without having to invent a new kind of matter. Perhaps studying gravity on the scale of our solar system only gives us an approximation of how gravity works. Perhaps there is some mathematical factor in the gravitational equations that only becomes measurable at galactic scales.

Right now dark matter is the dominant theory and MOND is a minority alternative. A new piece of data, however, may shift the balance a bit.

A thorough study of 28 galaxies has uncovered a curious feature – all of the galaxies (despite their age or shape) have “five times more dark matter than normal matter where the dark matter density has dropped to one-quarter of its central value” (according to New Scientist). Dark matter theory cannot account for this remarkable mathematical symmetry. Such symmetry implies that we are dealing with a force that has precise mathematical features. There is no property of matter that can explain it.

In other words, this feature might suggest that MOND is correct – we do need to tweak the gravitational equations, and if we do it in the right way we might not only explain the “extra” gravity we are seeing but this curious symmetry also, and dark matter will go the way of the ether – an invention to help explain cosmology that turned out to not exist. It will be fun trivia for science buffs in about a century, though.

Of course, this observation, while incredibly interesting, is not enough to kill dark matter. There are still other lines of evidence, like the bullet cluster, that tip the scales toward something like dark matter as the explanation. But perhaps this new observation will give MOND more life as an alternate theory. Or perhaps there is some third alternative no one has thought of yet. Or perhaps there is some truth to both theories – there is some dark matter but our gravitational models also need tweaking.

I’m not currently taking a position on this controversy. I’m just standing on the sideline watching astronomers fight it out over their competing theories. It’s like a science soap opera, although the story unfolds much more slowly.

So far, each new piece of evidence has seemed to deepen the mystery, raising more questions than they answer. That is also often typical of how science progresses. In the early phase of any new theory each new bit of data increases the complexity and confusion of the question, until we get enough pieces to start to put the big picture together. We are right about at the point of maximal confusi0n with dark matter, or at least it seems that way to this neurologist. I’m happy to be corrected by any astronomers out there.

We still have no idea what dark matter is. Until we solve that puzzle, I suspect that MOND theory or something like it will persist. It may even turn out to be true.

I can’t wait to see how it all turns out.

22 responses so far

22 thoughts on “Is Dark Matter Real?”

  1. Brian English says:

    Very cool. It will be great to see whether dark matter goes the way of epicycles and phlogiston or turns out the to be part of the fabric of the universe. The best bit, is that no matter how hair brained the explanation, if it doesn’t explain and doesn’t predict better than the competitors it’ll loose out. It reminds me of an article I read the other day, where the author had already decided that science could never explain consciousness and that evolution that didn’t lead to us as it’s outcome was just ultra-darwinism. Why did it remind me? Because one is science and worthy, the other is dogma and unworthy.

  2. Brian English says:

    that was “its outcome” not “it’s outcome”. D’oh!

  3. caoimh says:

    Ethan Siegel over at ScienceBlogs has a four part blog entry on the subject here:


  4. Michael Hutzler says:

    This is an excellent example of how proper ad hoc hypotheses differ from the ad hoc form of special pleading. Upon finding that the current model and observations are not compatible, both the model (Newtonian dynamics) and the observations (or limits of detection, in this case) are challenged with new testable hypotheses that can be accepted or rejected based upon future observations.

  5. Does the term “MOND” bother anyone else? Surely, for calculations this detailed and at this scale, astrophysicists use general relativity rather than Newtonian dynamics. So, I would think they’re modifying general relativity rather than Newtonian dynamics.

    Or, am I wrong? But, if I am, why the heck aren’t they using the full equations if they’re so worried about inaccuracies? Shouldn’t step one be to remove the simplifying assumptions?

  6. artfulD says:

    Isn’t there an underlying assumption here that dark matter overall will have the same consistency of properties that normal matter has been found to have when it comes to quantitative measurements? If so, wouldn’t the tweaking need to start with that assumption?

  7. sonic says:

    cheglabratjoe is right. But I think the Newtonian approx. is easy to calculate and is considered close enough (in that the answers using gen relativity would leave the same problem)
    Ease of calculation is a big deal– Ptolomy was more accurate than Newton on predicting planet positions, for example (little known fact)

  8. sonic,

    Well, ease of calculation is a big deal if by “not easy” you mean “requires a supercomputer.” I’m sure Excel on a laptop could crunch either Ptolomy’s or Newton’s equations all-but-instantaneously.

    Even if using relativistic equations for these calculations is as computationally expensive as you’re implying, you’d think someone would’ve applied for and received the necessary time on a supercomputer. This is a pretty big deal, right?

    I think the most important point is that tinkering with general relativity gives you a much better acronym: MOGRE (MOdified General RElativity).

  9. tortorific says:


    The main difference between Newtonian gravity and general relativity (GR for short) is that GR is time dependant. In order to use GR to work this out you need to trace every single planet back in time to the point when light from the planet would be reaching the target now (sorry I’m a physicist not a writer).

    Think of an expanding bubble moving at the speed of light back in time. Whenever there is something on the surface we add the gravity from that. You need to do this from every object in the galaxy to every object in the galaxy and each object will have a different position and target depending on the original target. We simply do not have enough super-computers.

    What we end up doing is approximating what is going on so in the end we aren’t using true GR or true Newtonian gravity, but which produces a result close to what is really going on. The errors we are seeing are much much higher than could be caused by our approximations.

    As for why we call it MOND the guy who initially proposed it wanted the modification to momentum not to gravity, I think perhaps the name has stuck as it is a little catchy. Most theories these days are about making alterations to the formula for gravity that through some mathematical trickery are only visible at long ranges. The formula Einstein used is almost identical to the formula Newton derived, the only difference is that you measure the distance not to where the object is now but where it was when it sent out the gravitational signal (which travels at the speed of light).

    This is a very believable theory, especially as we are currently going off the assumption that space is permeated by a scalar field that affects all matter with mass (the Higgs field) Newton derived his law from geometrical properties of space, if the Higgs field exists then it could very easily interfere with gravity.

  10. wb4 says:

    Could the Pioneer anomaly be another piece of this puzzle? The probes are slowing down slightly more than engineers predicted they would as they move away from the sun. If the force of gravity falls off with distance more slowly than we thought….

  11. cottreau says:

    After reading Lee Smolin’s “The Trouble With Physics”, I was left with the distinct feeling that dark matter doesn’t exist.

    Although MOND doesn’t quite explain everything, it might well be that they don’t have the math right yet – Newton didn’t have his math exactly right, so it’s forgivable.

    The Pioneer probes mentioned by wb4 are off by the same factor as gravitational factors at the edge of galaxies. Apparently, objects that are accelerating at incredible slow rates are affected by this phenomenon at an acceleration threshold – it seems somewhat arbitrary, but lots of natural phenomenon are (most universal constants for example).

    I’m feeling the same as you are – I can’t wait to see. I am betting on dark matter goes the way of ether.

  12. Truckle says:

    I’m with a lot of you guys on this, I have always been skeptical of Dark Matter/Energy…

    Its always felt a bit like ‘well we don’t know so there is this stuff that we can’t detect in any way but changes the calculations and the theories we have so they now fit.’ I know they have been able to make some predictions and tests based on this, but it has always somehow felt like a ‘fudge factor’ to me.

    Also when we are the ones poking at theists saying there is no way to detect your god, it seems a bit of a double standard to then turn round and say oh but there is this other stuff that affects the universe but we can’t detect it!

    Will be interesting to see how this pans out, if they can prove Dark Matter’s existence from this then fine, but until someone can tell me they detected some, i’m witholding my judgement.

  13. calinthalus says:

    I’m not a scientist. I don’t even play one on TV. However, I’ve never liked the dark matter hypothesis. I mean, calculations don’t work out as expected…so there’s something wrong. Do I assume there’s something wrong with the way my calculations come about…or do I invent an entire new class of matter of which I have no evidence whatsoever?

  14. tortorific, thanks for the detailed response. I think I see your point. I have a tangential question, though, if you don’t mind my asking. You always hear that one of the “proofs” of GR was that it successfully predicted the precession of Mercury. Was that prediction only by way of an approximation of GR? It must have been, according to what you’re saying, right? (I’ve never done any work on GR, in case it’s not clear. I took an intro to modern physics class in undergrad, but it only covered SR.)

    wb4, I’ve also wondered if the Pioneer anomaly could be explained by this. cottreau, do you have a source for the claim that the probes are off by the same relative amount. (Not calling you out, just wondering.) I sometimes hesitate to bring up the Pioneer issue, because it seems to bring the crazies out of the woodwork. I’ve dealt with folks who can only be called modern physics deniers in the past, and they LOVE the Pioneer anomaly.

    As for dark matter as a whole, I’ve always found it fishy. Most of my knowledge is secondhand, though. I just always got the impression that all the evidence for it that they have is just as well explained by gravity being off at large distances and/or times. For instance, my ears always perk up when Pamela Gay goes over the evidence for dark matter on AstronomyCast … and then I just think to myself “couldn’t gravity being wrong explain all that just as well.” And then Frasier mentions that “we have a map of dark matter,” and I think “well, that’s either a map of dark matter, or a plot of error vs position.”

  15. dcardani says:

    Truckle said:
    “Also when we are the ones poking at theists saying there is no way to detect your god, it seems a bit of a double standard to then turn round and say oh but there is this other stuff that affects the universe but we can’t detect it!”

    I don’t think that’s a fair assessment of science. Scientists are saying, “We think this is how it works,” and then they test it and it’s correct up to a certain point. So they look at it and say, “Well, it’s not right on this really large scale, what are the possible reasons?” And they come up with more tests. That’s what we’re seeing going on today. Some scientists are testing the dark matter theory, and others are testing MOND. (And there may even be others testing something else.) In the meantime, people who need to get work done can use whichever model gets them the best (i.e. most correct) results for their work, until we can figure out which is correct. And that work may give further information telling which is more correct, too.

    This is completely different from theist who often say, “Well, I can’t explain this, and I don’t feel like trying, so God did it!” Since science is always provisional, scientists aren’t really saying, “It works this way.” They’re saying, “As far as we can tell, it works this way, and here’s how accurate our model is, and here’s how we know that.” Whereas, theists often say, “It works this way because someone claimed it does.”

    If you don’t think dark matter or MOND are correct, you can go test them yourself. If you don’t think a particular religion is correct, there’s often no way to test that.

  16. tortorific says:

    cheglabratjoe, For mercury, kind off, gravity drops off as distance squared and the distances between planets and the sun is much much smaller than the distance between stars. We can safely neglect the gravity from the rest of the galaxy then as equally effecting all elements of the solar system. Then we can take the plants and the sun and work out the position using the proper general reletivity, even adding the moons to the mass of the planets and approximating them as a homogeneous blob of mass (same for the asteroid belts) the time factor still makes it a difficult problem for a supercomputer with just the planets and asteroid belts.

    Dark matter is still more likely than MOND, GR and newtonian gravity are based on geometric properties, you would need something weird going on for geometric properties not to hold (like a field that permeates the entire universe similar to the ether). But at this stage it could be anything, we really have no idea.

  17. tortorific, thanks again for the responses, they’ve been great and informative.

    One last question for you: could the “weird thing going on” be vacuum fluctuations? I have to imagine that someone’s checked for this, but it (naively) seems to me that this all might be a gigantic example of the Casimir effect. So, I guess what I’m asking is: could dark matter just be virtual particles?

  18. Myrrdin64 says:

    Very interesting! I’ve always been of the opinion (and I am in NO way, shape or form even remotely qualified to have an opinion on this topic) that the idea of dark matter felt … contrived. I’ve followed the ideas of MOND from afar. I, too, can’t wait to see how it all turns out!

  19. FYI – I interviewed an actual dark matter specialist last night. You can hear the interview on the SGU on Saturday (I will post an addendum with a link).

    He clarified for me the point about the Bullet Cluster. What this shows is that the bulk of the gravity of the clusters separated out from the bulk of the normal matter (the gas and dust clouds – the stars actually have minimal mass percentage wise).

    MOND cannot explain this – gravity separating from matter. Dark matter can – because the gravity is actually dark matter that can separate out from the gas and dust clouds, because the gas clouds collide and slow down, while the dark matter passes right through.

    This sounds like pretty much a slam dunk for dark matter.

  20. artfulD says:

    Assuming you are limited to those alternatives.

  21. HolidayNova says:

    Lets go back to what we know about matter. We have particles that interact electromagnetically and we have neutral particles that don’t interact. We have mass objects that curve space/time. In the bullet cluster if this invisible mass which is the majority and “invisible” why can’t they just be caused by majority of some neutral particle like lots and lots of neutrons! enough would interact gravitationally maybe with some seed like collapsed stars. Or could it just be collapsed stars probably black holes because anything smaller would llikely still be around because the universe doesn’t seem to be old enough for them to collapse yet. I wonder if space neutrons exist? It could be a definite candidate for a theory to be modeled.

  22. HolidayNova says:

    Neutrons only live for about 15 minutes when they aren’t next to a proton. So dark matter can’t be neutrons as we currently understand it.

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