Darker Matter

In Dark Matter by Brian Koberlein15 Comments

There’s nothing like dark matter to fire up a discussion about astrophysics. We have lots of evidence to support dark matter, and we’ve determined some of it’s physical properties such as the fact that it is cold rather than hot (ruling out things like neutrinos). But because we don’t know everything about dark matter, and since we haven’t directly detected dark matter particles in a lab, there are constant accusations from the general public that dark matter is just a “patch” over our ignorance, or that astronomers have simply made up dark matter in order to keep the status quo (and our jobs) alive. If we can’t see it, the argument goes, then we can’t be certain it exists. Except we can see it, such as the blue smudges in the image above.

We can't see infrared light either, but we know it exists.

We can’t see infrared light either, but we know it exists. Credit: Spitzer/Kitt Peak National Observatory

The image shows a false-color representation of dark matter in six different galaxy clusters. Just as we can’t see infrared directly, but can detect it’s presence through CCD camera, we can detect dark matter by its gravitational effects on background light. The images are part of a recent work that analyzed the dark matter distribution in 72 colliding galaxy clusters. The results have further defined the nature of dark matter. For one thing, the distribution of regular matter and dark matter are distinctly different in these clusters. So much so that the team can validate the existence of dark matter to 7.6σ. In other words, the chance of these results being a false positive is 1 in 3 x 10-14. It’s hard to get more certain than that, so if anyone wants see direct evidence of dark matter, show them the above image.

But the results didn’t stop there. Because these are colliding galaxy clusters, the dark matter from each region is colliding as well. Since dark matter is diffuse rather than clumped into stars, when dark matter collides it will interact, and the strength of this interaction can be determined by the resulting distribution. We measure the interaction strength of particles as a cross section, and this work shows that the self-interaction cross section for dark matter is less than 0.47 cm2/g. This is quite small, and it means that not only is dark matter weakly interacting with regular matter, it’s weakly interacting with itself. Basically, it’s even darker than we thought.

This constraint is strong enough that it eliminates some of the models trying to extend the standard model of particle physics. It also puts into question some of the claims of gamma rays being produced by colliding dark matter. There are still things to learn about dark matter, and we’d still love to detect dark matter directly, but the claim that dark matter might not exist is untenable at this point.

Paper: Harvey et al. The nongravitational interactions of dark matter in colliding galaxy clusters. 347 (6229): 1462-1465 (2015)


  1. Inasmuch as we don’t have a clue how gravity works, (it’s too weak to be what it is) and we can’t even verify that the Universe is speeding up, (it may be a localized collapse in our individual area) you can’t use this data to ‘prove’ dark matter exists, and you certainly cannot presuppose that it acts to ‘hold’ galaxies together, (it may be that gravity has another component, which we’ve missed by trying so hard to prove dark matter)

    1. Author

      Actually, we know quite well how gravity works. We can verify the the universe is accelerating, and we know dark matter exists. If you think the evidence isn’t there you haven’t been paying attention.

      1. I think the gravity works two (+ time) dimensionally in the galaxy scale. Then the gravitational force depends not on (1/r^2) but (1/r), so the galaxy rotation curve becomes naturally flat.

  2. This is very interesting.

    I am interested to know how much dark matter was produced after big bang. It shouldn’t be difficult to find considering it weakly interact with itself. Also, did dark matter originated with 4 fundamental forces or was it already existing.

    Dark matter and Dark energy might be smoking gun for inflation before big bang rather than Gravity and Gravitational waves (Gravity works on ordinary matter and there was none before big bang)

  3. I’m not sure I understand this post.

    Why do the stars in two galaxies clump in the middle after the galaxies collide?
    If the clumping is caused (almost) only by gravitational interaction, the dark matter halos should clump approximately the same way.
    So is the the leading theory that the clumping is caused largely by friction and star formation, with the slowed-down parts pulling the the faster stars?

    If so, then this estimate of self-interaction among the dark matter only applies to the part of it that is gaseous.
    What if a large part of dark matter in rock-like form, made from non-baryonic molecules held together by an unknown dark-matter-only attractive force? Wouldn’t this give the same final dark mass distribution as the model of gas with low cross-ratio?


    1. I think the standard answer (I am not an expert) is that charge is what causes normal matter to stick together and dark matter particles have neutral charge, like neutrons (but of course free neutrons have a very short half-life so DM is not neutrons).

  4. Care to explain the 20+ parameters (and counting) that have needed to be added onto the big bang theory, the only theory that predicts dark matter? New science is slowly showing it’s more likely that there is no singularity. Keep up with the times.

    1. “20+ parameters”? Would you mind saying where you got that from?

      “the only theory that predicts dark matter”? I didn’t know that! I thought DM is just a parameter in standard cosmological models. Again, may I ask you for a source?

      “new science …”? Really? What new science is that, may I ask?

  5. I am a layman, not a scientist, and I am not critical of Dark Matter as a theory, but I do find myself a little perplexed by the “it is as real as infrared radiation” claim in this article. I would love to understand the data for Dark Matter more.

    This article says that I “can’t see infrared” and that’s similar to how I can’t see dark matter. But that’s not really true at all. Forgetting the fact that I can feel infrared (because the same is not true of ultraviolet) I can actually see infrared – just not with my eyeball. As the writer pointed out, we can all see infrared with the right equipment. A solider can see infrared and shoot someone. It is not visible to the human eye, but it is otherwise “directly visible”.

    Dark matter, as far as I understand it, is only visible by the effects the occur in it’s (theorized?) presence. Am I wrong about this? Is there a device or experiment that can actually record/see dark matter like a camera can see infrared, or ultraviolet, or radio waves for that matter?

    1. Author

      I did a series of posts a while back on the evidence for dark matter. To answer your question “can we actually record” dark matter, the answer is yes. We do it much the same way we create radio images. Radio antennas just detect radio waves from a particular direction. By gathering a whole bunch of measurements of an object we can map the object to produce an image. We can do the same for dark matter. Look at the gravitational lensing over a region of sky and we can map dark matter. You might argue that it’s still “based on theory”, but that’s true of everything, including what we see with our own eyes. What our eyes detect is used to create a model of what we see. Dark matter is indeed very real.

      1. Well, ovidius has a point though. Is dark matter not like plato’s shadows? All we see is shadows, but somehow we can’t replicate those shadows. With radio waves, we’ve been able to control that energy to create radios, tvs, walkie talkies, baby monitors, cell phones, CB radios… the list is very long. Infra-red, ultra-violet, x-rays, gamma rays, and even electrons… we’ve been able to empirically predict and use to our benefit, able to mold the universe to our will… What have we been able to do with Dark Matter? How is it tangible in our every day lives? All of the other things that have been discovered are much, much more tangible, and therefore, more real. Dark Matter seems to be like the sun setting. We only see that it sets, but haven’t figured out yet that our planet is round and it actually spins on its axis, giving the illusion of a sun rise and a sun set. Gravity is the same way. We’ve been using it since the day we rolled a rock down a hill, built aqueducts, etc. But, we’re still not able to explain the energy that displays it. But, we’ve been using it. I remember reading once about how Dark Matter was simply used to explain things that we couldn’t otherwise. Doesn’t sound like sound theory to me. Sounds like a cop-out. As ovidius has said, we can roll a ball down a ramp and see gravity, warm up water in a microwave… but somehow, with dark matter, there’s no evident data.

        1. Author

          No, he doesn’t. Basically you’re arguing that the application of a certain phenomenon toward human technology is the only true knowledge, which isn’t so. For example, we’ve been baking bread and brewing beer for millennia, simply as a matter of following a set of instructions without any real understanding of the underlying mechanism. True, that is some level of understanding, but not a deep one. With the development of germ theory and a modern understanding of yeast we do know the underlying mechanism. Our understanding is now much deeper. On the other hand, we know how the Earth orbits the Sun, and we know in principle how its orbit could be changed, but we don’t have the power to change Earth’s orbit. You can have power without understanding, and understanding without power. Dark matter is in the latter category. We understand it. We can confirm its effect on light and other matter, but we cannot control it. That’s not a cop out, that’s science.

  6. Brian, the photos are wonderful. Given that dark matter is observed with gravitational lensing, how does one know that the lensing is not due to a thin gas or dust cloud between the galaxy and us? It seems that everybody is sure that the dark matter is near the galaxy. How does one know that so precisely, when looking at the lensing effect?

    1. Author

      The effects of gas, dust and plasma on light are measurably different than the effects of gravity, and they don’t create this kind of lensing effect.

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