It's actually the opposite. It's observed by the effect it has on gravity in the region. It's explicitly not observed by blocking or emitting photons, but that's only one way of observing things.
Dark matter's effect is only needed because there are other factors that are presupposed. If you set the age of the universe at 13.787 Billion years old (oh wait, now they think it's 27.3 Billion now), then the large scale structures we see in the universe make no sense.
So to my point, we are not "seeing" dark matter anywhere at all... What we have is a failure mode in our current models to explain the phenomena we see. And to fix it they plug in magic invisible heavy stuff. Literately a chunk of math to balance equations in the rotations of galaxies.
Yes, cold dark matter is something conjectured to explain what we see, because we see these irregularities and we need something to explain what is causing them. If we weren't seeing those irregularities we wouldn't need it.
>> What we have is a failure mode in our current models to explain the phenomena we see.
The thing is, you don't know that, and that concept has already been thought about many times over.
What is surprising about CDM in particular is that it is ridiculous and yet it survives observation and theory better than every other hypothesis to date. It is more intuitive that the mathematical equations are wrong, or they've measured something (like the age of the universe) wrong, or there's some kind of effect that hasn't been accounted for. Yet after testing all that, CDM fails to be thoroughly disproved. The vast majority of evidence points to it being a real thing. If it looks like a duck and it quacks like a duck...
I don't think it's appreciated enough that astrophysicts do know that this is weird. That's why it's so interesting.
However, you need to come up with a competing theory that actually holds up to rigor. It's no good saying "it must be this" without any supporting evidence to support it. It's simply a guess, sci-fi, not science.
Yes, it's entirely possible that it will be explained by some part of physics we currently have no knowledge of. Some addition to the standard model, perhaps, or some miniscule effect that falls out of quantum gravity, whatever that may be. But those things aren't even a hypothesis at this point, just pure speculation without justification.
