Fermi may have spotted dark matter

By Phil Plait | November 19, 2009 8:00 am

One of the secondary goals of the Fermi gamma ray satellite is to look for the signature of dark matter. One idea for dark matter is that it’s composed of weird (and as yet undetected) particles called WIMPs (weakly interacting massive particles). A very odd property about them is that they are self-annihilating: when two of them touch, they turn into energy (and other, more easily detectable particles). When I first read about this several years ago I was pretty excited, because this is finally a testable hypothesis about dark matter.


My fellow Hive Overmind blogger and astronomer Sean Carroll writes that it’s possible Fermi has done just this. The data are not conclusive, but very provocative nonetheless. He has the details.

But I can’t resist adding that on The Big Bang Theory a few weeks ago, Raj and Sheldon were investigating building a detector to look for this very type of dark matter. I wrote David Saltzberg, the science advisor (whom I met on the set last month when I was visiting LA; more on him and that at a later date) and told him this, and he noted that I was right. Well, how about that! It had to happen sometime. Now, to publish…


Comments (39)

  1. Jack Mitcham

    I’m kinda rooting for the supersymmetrical counterpart to the neutrino, the neutralino, to turn up some experimental proof.


    That seems to fit the characteristics of Dark Matter.

  2. What, no news of today being the 40th anniversary of the Apollo 12 EVA?

    Oh, and this is very, very cool.

  3. Steve in Dublin

    And in further news… in the real world, when two wimps meet… nothing much tends to happen, really. Unless one of them wants to make something of it 😉

  4. Steve in Cornwall

    I have a gut feeling that Dark Matter and Dark Energy are the Epicycles of modern Cosmology, and that it’s more likely that our models are wrong.

  5. Jack Mitcham


    All models are wrong. Some are useful.

  6. KCS

    Please forgive my ignorance, but if dark energy and matter make up 95% of the universe and supposedly have a big impact on gravity, why are they so difficult to detect? Isn’t dark matter responsible for the outer galactic stars speed, then shouldn’t our outer planets also be orbiting faster? Thanks, KCS

  7. enigma3535

    This may be a naive question but it has been bothering me for a while and I can’t seem to figure it out on my own so I was hoping someone could clarify what I am missing … some time ago, the apparent misuse of the scientific term “theory” by creationists in their propaganda led me look into what “theory” actually meant in a scientific context. Some time later, I saw a program or read an article about the movement of galaxies in the universe and how the “law” of gravity did not appear to explain what was being observed so 2 variables [dark matter and dark energy] were postulated to solve for what was being observed. This struck me as effectively downgrading gravity from a “law” to a “theory”. What am I missing?

  8. Jack Mitcham

    You’re missing the fact that humans are fallible biological organisms, and not hyperlogical robots. People are sloppy with language. That’s why we have Newton’s LAW of gravity, which is demonstrably false, and String “Theory,” which doesn’t really even have a way to be tested yet.

    It isn’t like there is a strict hierarchy.

  9. As near as anyone can tell, nature operates by laws. There are strict, complex rules she follows. But the only way humans can learn about those laws is by observing nature and trying to figure them out. Our understanding of nature will never be perfect, so our knowledge about ANY natural subject will only ever be a “theory.” In some cases, our theories are quite mature and robust, and match what we actually observe in nature just about perfectly. In other cases, our theories have some problems that require further refinement. And of course, some theories are just plain wrong.

    But there’s never a point at which our theories are “upgraded” to laws. At best, they are very close approximations of natural laws.

  10. (Speaking as an astrophysicist, but not a member of the Fermi Collaboration.)
    Perhaps this was said on Sean’s post, but its worth repeating here. 1.) There are still some questions about the analysis, and disagreement about if this gamma ray halo actually exists. 2.) The original authors (http://arxiv.org/abs/0910.4583) only say that the haze is caused by a population of relativistic electrons. Dark matter annihilation is only one, exotic, explanation.

  11. Brian

    “Law” and “Theory” aren’t really members of a hierarchy; they are usually two quite different things. A law typically refers to a relatively concise description of observed behavior, while a theory is an attempt to describe the underlying mechanisms.

  12. Sili


    And most people’s gut-feeling tells them that the Sun goes around the Earth. You can’t trust your gut.

    Poke around Ethan’s blog for a thorough, but eminently readable summary of what we know of DM and DE.

    Also, epicycles are pretty damn hard to work with – remember there was only geometry, no calculus in those days. It was indeed a useful model until it was superceded. As of yet noöne has come up with anything that even remotely looks like a ‘Dark Ellipsis’ to explain the objective observations that force us of conjecture DM and DE (two very different subjects, incidentally, that cannot be dismissed in one breath).

  13. IBY

    @Steve in Cornwall
    But so far, those two are the only models that have worked so far on explaining the structure of galaxies and the expansion of the universe respectively. And note, dark matter and dark energy, there are various candidates for them, so we don’t know exactly what they are, and so we still don’t have a very good model on those. But based on available evidence, there seems to be more matter than are seen, and some kind of energy that is making space itself larger.
    If you are curious about dark matter, this post of an astrophysicist: http://scienceblogs.com/startswithabang/2009/10/what_is_dark_matter.php, which has the links to his dark matter series, explains why there has to be dark matter.

  14. IBY

    Arrgh! Looks like Sili has reached before me in response.

  15. Throwing in my $0.02 on Law/Theory, as I understand it….

    LAW is how things work
    THEORY (please capitalize the T) is why they work

    Remember, this is a ‘kindergarten’ explanation….


  16. Gary Ansorge

    Ah, Dark Matter is just a bunch of left over neutralinos(super symmetric partner to the neutrino). Certain GU theories predict these in about the quantity we seem to observe.

    On a brighter note, for beauty, check out this link my Son left me on Facebook. It shows how a ring system might look if earth had one.


    GAry 7

  17. Gary Ansorge

    14. John Paradox

    Good description. Another explanation is:

    LAws are a mathematical description of the way the universe behaves/operates, as in,
    S=1/2 a*t^2
    or F=G*M1*M2/R^2

    or perhaps the most famous observation of all:


    These are all just mathematical descriptions of what we observe, thus they are LAWS. They in no way explain HOW all that happens, merely that it does. From these and other observations we construct a THEORY to explain why things act that way. As noted, all theories are approximations, but some are really GOOD approximations.

    GAry 7

  18. Epicurus

    The definition of “theory” and their relation to “truth” is a topic of philosophy (epistemology to be more precise). Within natural sciences the currently dominating model is the so called “critical rationalism” by Karl Popper.

    In a nutshell a theory explains observations: Good ones explain a lot with a small set of variables, are free of internal contradictions and can be tested. A theory is not a description of reality: It is always temporary because the next observation can violate it.

    I always had a bad feeling about the concepts of dark matter and even more dark energy: Observations had been made which proved that current theories were wrong, but instead of taking this a a rejection of these theories, a speculative new variable had been introduced. The concept of dark matter explains several things (gravitational lenses, rotation speed of galaxies, etc.), but with dark energy we are close to the border of real natural science (and I’m not sure at which side).

    Popper introduced “falsifiability” as the criteria to distinguish a scientific theory from everything else. If a theory can not be proven wrong by observations, it is not science. I wonder if dark energy is a “testable” concept or only a gap filler in a model which did not stand the empirical test. Would be great to finally get some direct evidences which gives us the feeling to understand the world we live in a little bit better :-)

    Recommended readings: “Karl Popper”, “Epistemology”, “Falsifiability” and “Critical rationalism” on Wikipedia. Any recommendations from your side: Highly welcomed.

  19. I was always, at least in the past, on the MACHO side of the fence.

    A galactic halo peppered with innumerable brown dwarfs — which are probably MUCH more common in space than the few we’ve detected so far — would explain the Dark Matter phenomenon in a way that didn’t require any new and exotic kinds of matter to be invented.

    But, sadly (for me), as time has worn on, the WIMPs seem to be winning.

  20. Mike Wagner

    Off-topic but awesome:
    I just stopped by the library to grab some Nat’l Geos the librarian was getting rid of and noticed that it coincided with a view of the ISS, so I let everyone know and a bunch of people piled outside to see it.
    The first one to spot it was a little girl, probably about three, and she was really excited.
    Hopefully there’s going to be some scientists come out of that bunch of kids :)

  21. Torbjörn Larsson, OM

    “Finally I have seen the dark!”

    A very odd property about them is that they are self-annihilating: when two of them touch, they turn into energy

    Are you sure they are WIMPs?! 😉

    But really, this isn’t so odd, photons are their own antiparticles as well, and the putative gravitons too I guess. Particles must have antiparticles for reasons of symmetry, and some interactions doesn’t seem to leave much other options than annihilating “self”.

    This struck me as effectively downgrading gravity from a “law” to a “theory”.

    Effectively a law is an isolated hypothesis, which predict and can be tested on one thing only.

    A theory is by its nature a consistent set of related hypotheses, which can be tested on a multiplicity of predictions. Thus theories are the strongest knowledge we can have.

    So todays cosmology that incorporates DM and DE is stronger than mere GR cosmologies. In fact, I believe I’ve seen claimed that they admit testing GR, “gravity law” if you dump the fancy “general relativity theory” (curved spacetime and so on) beneath if you will and just calculate, out to scales never before attempted.

    For example, a detailed theory like evolution with its many subprocesses combining to the whole process of life, has made and tested predictions of every level of detail for 150 years now, published more or less daily as of today. To replace it we would need a theory that makes the same predictions but is simpler, or alternatively that makes at least one prediction more.

    That is a very, very tall order.

  22. albean


    Sorry, more people would remember if I had thought to turn the camera properly.

  23. orin

    @1, @15
    BTW, the neutralino is not the supersymmetric partner to the neutrino. That would be the sneutrino.

  24. PJE

    Is it wrong to remain skeptical about this? (For now)


  25. Steve in Cornwall

    @IBY MOND looks like the application of Occam’s razor in this case. I’m sitting in the “we don’t know” camp on this one, but MOND looks a much more straightforward solution than DM/DE – but I’m also sure MOND doesn’t give all the answers either. For the record, I’m also a superstring sceptic… :-)

  26. Kopeliadis

    I’m not a scientist so…
    Why is there so much (?) dark matter since WIMPs are “cold” (meaning slow) and “heavy”, thus clumpy, and self annihilating? Is that this annihilations is slow,that they are continously creatted or that the universe is still young?

  27. Sili


    Read Starts with a Bang. MOND actually complicates matters in that it would require gravity to be non-local. It may explain galaxy rotation curves, but that’s it. It doesn’t work for the bullet clusters.

  28. enigma3535

    Epicurus and Carl: “thank you”; unfortunately, I do not think your posts help regarding the dilemma … re: … Mr. Mitcham, IMHO, I am not missing out on the fact that “humans are fallible biological organisms, and not hyperlogical robots.” Nor that, “People are sloppy with language.” I am concerned that there are materially important issues facing both the governance of society and the education of our children that are being impacted by the words used in our scientific community. I am chagrined that there does not appear to be logical standards regarding what is a scientific “law” vs “ theory”; and, this lack of a standard nomenclature *is* – with emphasis – being used against my kid’s future by persons that I am almost certain know better.

    Torbjörn Larsson: I think I understand most of what you are saying, but, it does not appear to help very much outside a select group of people that “get it”.

  29. davem

    So if we know nothing about Dark Matter, how come we know that its particles annihilate each other? that sounds like we know a lot about dark matter to me…

  30. Hypocee

    …finally a testable hypothesis about dark matter.

    Er…the Bullet Cluster…?

  31. Hypocee

    I don’t know how much Phil studies the quantum mechanics, but upon reading the actual article the definitive statement that “[WIMPs] are self-annihilating” was a misrepresentation. The actual statement made is that if WIMPs happen to be self-annihilating, then the galactic center would be the best place to look for such collisions.

    According to the abstract the data suggest that we may see a haze of otherwise-inexplicable gamma rays around the galactic center, and that if it is not some kind of artifact its spherical shape seems to contraindicate an origin in baryonic matter. That last one seems a little silly to me given that the baryonic matter near the center of spiral galaxies usually forms a sphere, but I presume that’s because they’re pro astronomers who spell out the reason in the paper and I’m an abstract-reading fanboi.

  32. astroquoter

    @ 8. Jack Mitcham Says:

    You’re missing the fact that humans are fallible biological organisms, and not hyperlogical robots. People are sloppy with language. That’s why we have Newton’s LAW of gravity, which is demonstrably false, and String “Theory,” which doesn’t really even have a way to be tested yet.

    It isn’t like there is a strict hierarchy.

    For illustration here & in historical order we have :

    Kepler’s *laws* of gravity ->

    -> explained by Newton’s *laws* of gravity ->

    -> refined by Einstein’s theory of Relativity ->

    -> confused by numerous theories of super strings and quantum physics and M-brane theory and WIMPs and Higg’s bosons and .. who knows what else they’ll think of next and we’re still really struggling to understand it all.

    Seems to me that somewhere along the line shortly after Einstein, physics just went and totally lost the plot…

    Or maybe its just me? :-(

    (I find cosmology confusing as all hell & my maths is pretty lousy. I am interested and read a lot about it – incl. here – but I still can’t seem to make much sense of it.)

  33. StevoR

    Neat news although cosmology ain’t really my thing and I won’t pretend to be on top of its subtleties. Fermi is the new name of the GLAST probe the BA worked on right? :-)

    @ 16 Gary Ansorge :

    Awesome link there thanks! 😀

    Except it gets me wondering could Earth have rings like Saturn with a Moon like ours or would the Lunar gravity disturb and destroy the rings via tidal effects?

    Stunning & well explained clip anyhow.

  34. astroquoter

    Deleted by author. Sorry.

  35. Petrolonfire

    @ 31. Hypocee Says:

    …finally a testable hypothesis about dark matter.

    Er…the Bullet Cluster…?

    Nah, I think that “bullet cluster” one got shot down – despite being pretty quick on the draw! 😉

  36. Hypocee

    ?! [citation needed]

  37. Hypocee

    Astroquoter, a shorter way to put your statement about physics ‘losing the plot’ is this: “I am surprised that strong, simple relationships got discovered sooner than subtle, complicated ones.”

  38. Fucious

    I remain very skeptical of dark matter.

    I’d like to see an explanation for the weak gravitational interaction of dark matter. If this is the case, then comparatively speaking, isn’t the dark matter accelerating differently due to gravity when compared to known matter? I mean this in the curved space-time sense of acceleration; so a hypothetical “dark photon” wouldn’t travel in a strait line through curved space-time, but wold instead curve away from gravitational sources.

    Or would dark matter exist in a parallel space-time overlapping ours where the curvature of space-time isn’t as severe? A model where different kinds of matter react to gravity differently would suggest this, I think. And I find that very hard to swallow.


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