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I have read this question:

As I understand, dark matter theoretically only interacts with the gravitational force, and doesn't interact with the other three fundamental forces: weak nuclear force, strong nuclear force, and electromagnetism.

If dark matter only interacts with gravity, why doesn't it all clump together in a single point?

There are a few questions on this site about dark matter annihilation and anti-dark matter, and all of them take it for granted that dark matter annihilates into photons.

Is there a possibility of anti-dark matter?

Dark Matter gamma-ray flux from hadronic annihilation channels?

How convincing is the evidence for dark matter annihilation at 130 GeV in the galactic center from the Fermi Satellite data?

Now as far as I understand, ordinary matter and anti-matter can annihilate into photons because ordinary matter does interact electromagnetically.

But dark matter does not interact electromagnetically, hence, dark matter cannot annihilate into photons, dark matter's energy cannot be transformed into the energy of the EM field.

Question:

  1. If dark matter can't interact electromagnetically, then how can it annihilate into photons?
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    $\begingroup$ Often in these situations, the dark matter annihilates into some Standard Model particles besides photons, and those particles then decay into photons. Also, you should note that it's completely possible for dark matter to directly interact with photons weakly. When people say it can't interact with photons, they just mean we know it has to interact with them a lot less strongly than ordinary matter. $\endgroup$
    – knzhou
    Jul 21 at 22:44
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    $\begingroup$ 1. Ponder, by contrast, the following question: If neutrons can't interact electromagnetically, then how can a neutron and an anti-neutron annihilate into photons? 2. Since we do not know what dark matter actually is, it is not clear what constitutes an answer to this question: Any model of dark matter that includes annihilation into photons? An explanation of what specific model of dark matter each of the questions you link is talking about (it could be at least three different model!)? An explanation that this question is ill-defined because we don't know what dark matter is? $\endgroup$
    – ACuriousMind
    Jul 21 at 23:02
  • $\begingroup$ @ACuriousMind you are correct, and maybe I am wrong, but when these questions talk about dark matter, they assume elementary particles for dark matter. Neutrons are made up of EM charged particles, and I believe when a neutron and an anti-neutron annihilate, in reality quarks and anti-quarks annihilate, which are EM charged. $\endgroup$ Jul 22 at 1:03
  • $\begingroup$ @knzhou thank you, that could be an answer. $\endgroup$ Jul 22 at 1:19
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This has been addressed in Neutrino annihilation and bosons since neutrinos are a type of dark matter (they just aren't the dark matter we need to account for the extra mass of the universe).

You are quite correct that a neutrino and antineutrino cannot annihilate directly to photons but they can do so via a Z boson. We don't know what the dark matter particles are but we expect they will annihilate by a similar mechanism involving an intermediate particle.

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Since we don't know what the quanta of dark matter are, we also don't know whether dark matter can annihilate into photons. It is perfectly possible that dark matter may not be able to annihilate into photons at all. But, and as @JohnRennie points out in his answer, it is conceivable that dark matter may annihilate into photons. This could be through some intermediate particle similar to the Z boson as he says. WIMPs in particular are one of the popular dark matter candidates, and for WIMPs and other thermal relic particles one would generically expect annihilation into photons as well. Or, taking the case of axions as dark matter for example, this can happen through coupling of axions to photons directly. So, if somebody pretends that dark matter annihilating into photons is a given, then they're sweeping basic assumptions under the rug.

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