Finding Out What Dark Matter Is – And Isn’t

Astronomers using NASA’s Fermi Gamma-Ray Space Telescope have been looking for evidence of suspected types of dark matter particles within faint dwarf galaxies near the Milky Way — relatively “boring” galaxies that have little activity but are known to contain large amounts of dark matter. The results?

These aren’t the particles we’re looking for.

80% of the material in the physical Universe is thought to be made of dark matter — matter that has mass and gravity but does not emit electromagnetic energy (and is thus invisible). Its gravitational effects can be seen, particularly in clouds surrounding galaxies where it is suspected to reside in large amounts. Dark matter can affect the motions of stars, galaxies and even entire clusters of galaxies… but when it all comes down to it, scientists still don’t really know exactly what dark matter is.

Possible candidates for dark matter are subatomic particles called WIMPs (Weakly Interacting Massive Particles). WIMPs don’t absorb or emit light and don’t interact with other particles, but whenever they interact with each other they annihilate and emit gamma rays.

If dark matter is composed of WIMPs, and the dwarf galaxies orbiting the Milky Way do contain large amounts of dark matter, then any gamma rays the WIMPs might emit could be detected by NASA’s Fermi Gamma-Ray Space Telescope.

After all, that’s what Fermi does.

Ten such galaxies — called dwarf spheroids — were observed by Fermi’s Large-Area Telescope (LAT) over a two-year period. The international team saw no gamma rays within the range expected from annihilating WIMPs were discovered, thus narrowing down the possibilities of what dark matter is.

“In effect, the Fermi LAT analysis compresses the theoretical box where these particles can hide,” said Jennifer Siegal-Gaskins, a physicist at the California Institute of Technology in Pasadena and a member of the Fermi LAT Collaboration.


So rather than a “failed experiment”, such non-detection means that for the first time researchers can be scientifically sure that WIMP candidates within a specific range of masses and interaction rates cannot be dark matter.

(Sometimes science is about knowing what not to look for.)

A paper detailing the team’s results appeared in the Dec. 9, 2011, issue of Physical Review Letters. Read more on the Fermi mission page here.

12 Replies to “Finding Out What Dark Matter Is – And Isn’t”

  1. Isn’t that what the collider is bogged down with? Elimination that never ends?

  2. If dark matter is composed of neutralinos this would mean they are not Majorana fermions. Majorana fermions are their own anti-particle, which means they annihilate if they interact with each other. This is slightly disappointing, for this means a signature is absent and I always thought there should be some instance of Majorana fermions in the universe.

    The other candidate for DM is the axion particle. These are related to the graviton and also have a CP violating mechanism with respect to QCD. These particles though are very cold, having been frozen into a state of near zero energy. They could though prove to be an interesting candidate, for they have some connection to quantum gravity.


    1. Quantum Vacuum Polarization and Negative Repulsion / expansion towards the outer bound non-zero-point vacuum seem logical and my instincts say we DM/DE are not the localized phenomena many expect.. Happy to be proven incorrect, of course..

      1. I have doubts about these ideas. Vacuum polarization in QED computes the renormalized charge and mass and defines a running parameterization of the fine structure constant. It is a bit hard to imagine a large mass effect that would rise to DM has been missed somehow. This of course might be extended to gravity as well. However, since gravity is very weak this effect is only prevalent near the singularities of black holes.


      2. Hmm…aren’t “seem logical” and “instinct tells me” how scientists often come up with hypotheses for testing? Seems like we wouldn’t be nearly this far along if all science was logically derived from prior art.

      3. Hmm…aren’t “seem logical” and “instinct tells me” how scientists often come up with hypotheses for testing? Seems like we wouldn’t be nearly this far along if all science was logically derived from prior art.

  3. I think this is the paper they published.

    [Small world. The coauthor NASA quotes, Maja Llena Garde, works at KTH but has worked at Uppsala University; I believe we shared the same group room a few times when I was working there lately.]

    It isn’t too harsh of a constraint as of yet. Some but not all WIMP annihilation channels are excluded up to some 20-40 GeV*; supersymmetric neutralinos masses in at 100 – 1000 GeV. (And I note they are fermions, so Majorana to predict the sought for self-annihilation, which I assume LC’s cryptic first sentence refer to.)

    Wondering how string theory, the theory predicting neutralinos, is doing, I found this on twistor string theory. Instead of using Penrose’s causally explicit twistor space to describe gravity, string theorists can use it to describe unpacking of large dimensions.

    It moves quantum field theory into quantum mechanics without having to inject extraneous procedures of explicit quantization I take it: ” “It’s the first instance known of a purely quantum-mechanical theory,” Arkani-Hamed says.” In a very handwavy way, perhaps this explains it: while a light ray in spacetime corresponds to point in twistor space, a point in spacetime corresponds to a line in twistor space. Making nonlocality of quantum mechanics explicit, while at the same time obeying causality.

    So string theory plays nice now with both gravity (predicting gravitons) and electromagnetism (predicting QED, I take it).

    * Which is, I believe without having the energy to check it out, an improvement with an order of magnitude from the first ever similar exclusion from one or two galaxies alone. If it continues like this they will soon start to cut into the interesting region.

    1. What about this:

      Evidence for extended gamma-ray emission from galaxy clusters

      Where is found that “if interpreted as annihilation emission from supersymmetric dark matter (DM) particles, the data prefer models with a particle mass in the range 20-60 GeV annihilating into the b-bbar channel, or 2-10 GeV and >1 TeV annihilating into mu-mu final states ”

      “This results are consistent with those obtained by Hooper and Linden from a recent analysis of Fermi-LAT data in the region of the Galactic Centre”

      They are referring to this paper:

      On The Origin Of The Gamma Rays From The Galactic Center

      So dwarf spheroidals show no sign of dark matter, but the galactic center and some galaxy clusters show some results consistent with a dark matter particle with a mass of a few GeV, a result similar to some direct detection experiments.

      What do you think?

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