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Based on Galactic Gamma Ray Emission

My question is simply put. Although dark matter itself is almost a complete mystery, has anything been published which postulates the existence of anti-dark matter?

I am assuming, probably incorrectly, that the annihilation mention in the extract below to explain gamma ray energy levels, is due to dark matter, anti dark matter collisions.

One of the problems with this idea is that the gamma rays appear to emanate from the centre of galaxies, whereas I am under the impression that the majority of dark matter is either in the galactic halos or located a long way out from galactic visible regions.

EDIT Please see comment below on GR emission origin by Brionius. END EDIT

The Large Area Telescope on board the Fermi spacecraft looks out for the most extreme events in the cosmos. Launched 2008, it scans the whole sky for gamma rays in the very high energy range, from 20 MeV to about 300 GeV; a full scan takes about 3 hours. One of Fermi’s most interesting findings is an unexpectedly large amount of gamma-rays, ordinary light but at enormous energy, stemming from the center of our galaxy.

The Fermi gamma-ray excess has proved difficult to explain with standard astrophysical processes. The spectral distribution of the observed gamma-rays over energies bulges at about 2 GeV and it is hard to come up with a mechanism that produces particles at such high energies that prefer this particular spectral feature. The other puzzle is that whatever the sources of the gamma-rays, they seem homogeneously distributed in the galactic center, out to distances of more than ten degrees, which is about 5,000 light years – a huge range to span.

The most exciting proposal to solve the riddle is that the gamma-rays are produced by dark matter annihilation. Annihilation spectra often bulge at energies that depend on both the mass of the particles and their velocity. And the dark matter distribution is known to be denser towards centers of galaxies, so one would indeed expect more emission from there. While dark matter isn’t entirely homogeneous but has substructures, the changes in its density are small, which would result in an overall smooth emission. All of this fits very well with the observations.

For this reason, many particle physicists have taken their dark matter models to see whether they can fit the Fermi data, and it is possible indeed without too much difficulty. If the Fermi gamma-ray excess was due to dark matter annihilation, it would speak for heavy dark matter particles with masses of about 30 to 100 GeV, which might also show up at the LHC, though nothing has been seen so far.

I can't immediately see a duplicate of this question on the list below the title box.

Alternatively, because we know very little about dark matter, it may be an opinion based question and not applicable here.

I will delete the question, if I am wrong, in either of the above cases.

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Part of the LHC program is to find new particles that can be what dark matter is: very weakly interacting particles. This will show that an extension to the standard model would be correct. This will follow the pattern: "for every particle there exists an antiparticle", or a particle can be the antiparticle for itself, so yes, in this case there will be antiparticles to annihilate and give gamma rays according to whatever couplings the extended standard model theory will have establisehed.

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  • $\begingroup$ Thanks anna, as you are aware far more than I am, so far it's all based on totally, as far as I know, indirect cosmological evidence. It would be nice to have a dark matter particle "in a box", even temporarily, here on earth. $\endgroup$ – user81619 Aug 3 '15 at 7:18
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Yes, the article you linked and quoted is describing a proposed explanation for observed diffuse gamma ray emissions from the center of our galaxy. The proposed mechanism is that of dark matter/anti-dark matter annihilation in the alleged dark-matter halo in our galaxy.

However, you seem to have gotten a wire crossed - this has nothing to do with Gamma Ray Bursts (GRB). GRBs are short-lived, unbelievably bright gamma ray emissions. They have never been observed in our own galaxy, only in other galaxies. In fact, it is possible that a GRB directed at the Earth (and therefore directly observable) originating from within our galaxy could partially or entirely sterilize the Earth.

The gamma ray phenomenon this article is discussing is a source of gamma rays that is constant (not in bursts) which seems to be spread smoothly throughout the central part of the galaxy.

Another item of confusion that you may have is with the term dark matter "halo". The term halo in this case refers to a globular distribution that increases in density monotonically as you get closer to the center of the galaxy. Theoretically, the dark matter density should be highest at the center of the galaxy, which fits the gamma ray emission profile that Fermi observed.

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  • $\begingroup$ More research and possible rewrite, less rushing to the question, (which still stands I suppose, though I have a feeling it's opinion based). thanks very much for that. $\endgroup$ – user81619 Aug 3 '15 at 2:15
  • $\begingroup$ Sorry - not sure what you mean here. $\endgroup$ – Brionius Aug 3 '15 at 2:18
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    $\begingroup$ Sorry, I meant that dark matter is mysterious enough in its own right, so that postulating the existence of anti-dark matter is pointless until we know more about dark matter. $\endgroup$ – user81619 Aug 3 '15 at 2:27

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