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 ?
"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.
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!!!!)
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.
