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From what I understand dark matter is called dark since it cannot be seen in the universe.

My question is does it have to be invisible, as in to not interact with light ?

Could it be that dark matter interacts so weakly with itself that there is never enough of it in one place for us to "see" it ?

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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/4652/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Feb 5 at 17:33
  • $\begingroup$ There is no evidence that dark matter is "matter" at all in the normal definition of being made up of atoms and molecules. The only thing we know is that DM generates gravity. There are theories that DM is more likely the excitation of a quantum field (similar to the magnetic field) which leads to gravity. $\endgroup$
    – foolishmuse
    Commented Feb 5 at 18:25
  • $\begingroup$ Relevant technical term: "millicharged dark matter" $\endgroup$
    – Sten
    Commented Feb 5 at 19:42
  • $\begingroup$ @foolishmuse All matter is "the excitation of a quantum field." That does not distinguish dark matter from electrons and protons. $\endgroup$
    – AXensen
    Commented Feb 6 at 2:42
  • $\begingroup$ @AXensen You are correct. But I was implying virtual particles, such as the magnetic field surrounding a bar magnet. $\endgroup$
    – foolishmuse
    Commented Feb 6 at 16:13

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"Dark matter" refers to the missing mass required to explain the observed motion of astronomical objects (e.g. galactic rotation curves). It does not necessarily refer to a particular type of material. Black holes in certain mass ranges have not been disproven. However, the most widely accepted explanation is a new particle (e.g. WIMPs or axions) which has little or no interaction with light or baryonic matter.

Diffuse matter which interacts with electromagnetic radiation as strongly as regular matter (and is not "invisible" as you suggest in your question) would be observable via scattered light if it accounted for the missing mass.

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    $\begingroup$ Thank you for your reply, it fully answers my question. $\endgroup$
    – Jonathan
    Commented Feb 5 at 12:59
  • $\begingroup$ @Jonathan you should "accept" the answer then. The check mark symbol near the up and downvote buttons. $\endgroup$
    – AXensen
    Commented Feb 6 at 2:42
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That's a very pedantic way to see things, and most physicist would consider this a non-argument. (some call it fine tuning.) It is of course always possible to say that some interaction is very small, small enough to be within the experimental bounds.

So in the same manner as your question is posed, the answer is yes: We can never rule out a very small coupling with light. (Although mind you that this does not mean our theory will be anomaly free, this is something you need to check separately!!!!)

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    $\begingroup$ Sorry if the way I formulated my question offended you, it was not my intention. Thank you for you answer. $\endgroup$
    – Jonathan
    Commented Feb 5 at 12:56
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    $\begingroup$ I did not feel offended $\endgroup$
    – Confuse-ray30
    Commented Feb 5 at 13:14
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    $\begingroup$ I believe @Jonathan was referring to the commonly pejorative connotation of "pedantic." $\endgroup$
    – Gilbert
    Commented Feb 5 at 15:57
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    $\begingroup$ I'd have to disagree with this pretty strongly - I've made a career searching for hypothetical tiny interactions of dark matter with hadronic matter, have a look at ultra-light dark matter and bosonic scalar fields $\endgroup$
    – CharlieB
    Commented Feb 5 at 21:40
  • $\begingroup$ Yes, but that's not what the question is about. We can never rule out anything in the way this question is asked. Will the theta angle of qcd ever be confirmed 0? No. Will the charge of gravity ever be confirmed to be the mass in the force? No. So saying can something be so small that it is simply not observed is a non-question to me, since the answer will always be yes. And you're more aware than I am why these small interactions still need to be big enough to see observable condequences. (Such as in structure formation) $\endgroup$
    – Confuse-ray30
    Commented Feb 6 at 7:52
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Yep, dark matter is essentially invisible because it doesn't play ball with light the way everything else we know does. It doesn't emit, absorb, or reflect light, making it totally invisible to any of our instruments. We only know it's there because of the gravitational effects it has on things we can see, like galaxies spinning faster than they should. So, in a nutshell, it's the universe's own stealth mode stuff.

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