Well, the very reason for conceiving of dark matter is because there is evidence of gravitational effects from mass we cannot observe. So dark matter by definition must at minimum interact gravitationally, and be hard to observe by other means (electromagnetically, maybe even intent to the nuclear forces).

Black holes sans accretion disk clearly satisfy these criteria.

Dark matter does interaction with matter via gravity. It just doesn't interact via electromagnetism, the weak nuclear force, or the strong nuclear force. Our usual observation techniques with electromagnetism can't detect it.

Yes. In fact, this is how we detect DM, by measuring the speeds of the galaxies, rotating around the galactic core. Our calculations point to a slower speed than observed, thus implying some mass of "matter" which we cannot see.

As far as I understand it: All kinds of stuff is happening in the universe that can only happen if there's much more gravity than we can explain with the matter we can actually observe. So there has to be something more than the observable matter. But we can't find it, so the assumption was that it didn't physically interact like normal matter (except for gravity), otherwise it would give at least some indication that it's there. Like infrared or something, I dunno.

But if it turns out it's black holes, I'm guessing it would have physical interaction with other matter but still be hard to detect and not interact in the usual way we expect of normal matter.

All this is speculation though, and I don't really know what I'm talking about.

Ignorant "ELI5" question from a non-astrophysics major: Couldn't all this unobservable matter be just normal but so-far unobservable matter? Little planets and rocks all over the place that we just can't see yet because our telescopes aren't good enough? Why do we think it has to be some other "exotic" form of matter?

Well, I'm really not qualified to answer, but I think it's because regardless of how small each individual piece of "debris" would be, together it should still form a huge amount of matter. All matter gives of electromagnetic radiation. If there was normal matter there, we'd see the electromagnetic radiation, but we don't. So it has to be something else. It's kinda like how we can see those epic cloud nebula formations. Those clouds are basically just dust and gas and other tiny stuff. And it's not as clumped together as you'd think, but we can still detect it lightyears away.

There are other more complicated reasons but this is a pretty good answer. Basically that matter at some point must have been at the Big Bang and with all the heat generated around that time some of it must have interacted with the debris that GP is talking about. We don’t see evidence of that. It’s important to remember that though we as humans lose things all the time. The universe cannot. We got what we got.

No, we've tested for that repeatedly. That was, of course, the first thing scientists thought of to explain this excess non-luminous mass.

Could it be large transparent gas clouds? No, it turns out that gas clouds aren't completely transparent, and can be measured in various ways. We looked, we checked, such clouds don't contribute enough mass.

Could it be dust clouds? Dust clouds are not luminous but they do block visible light, so they'd be obvious. We looked, we checked. It wasn't dust clouds.

Could it be rogue planets or brown dwarfs? No, such objects would cause gravitational micro-lensing effects, which can be observed in dedicated campaigns. We looked, we checked. It wasn't planets or brown dwarfs.

The only theory that has withstood the last several decades of experimental observation and simulation has been the dark matter theory (weakly interacting massive particles at sub-relativistic speeds). It's the only thing that explains all the evidence across the board.

Could it be that our traditional models of motion and light propagation in extremely distant, never visited places are extrapolations in error?

It seems most astronomy theoreticians will rather not rock the boat, they will rather keep believing in invisible stuff permeating space everywhere with variable but exactly right density so it is consistent with star motions and images observed. I guess it's because stuff with infinity of degrees of freedom is traditionally much more secure paradigm to base a career on than to question the established beliefs in applicability of Newtonian mechanics/General relativity.

> "The only theory that has withstood the last several decades of experimental observation and simulation"

Which competing theories were put to stand the test of observation? What made them not stand well in light of observations?

> "It's the only thing that explains all the evidence"

There is little example to such a thing as scientific theory explaining all the evidence. But even if dark matter paradigm did do such a feat, it would not be a good sign for the truthfulness of it. Theories that explain everything are suspicious - they may be exercises in fitting and may not be so scientific or useful as competing theories that explain less, but better.

This is extremely ignorant. You are demonstrating you have absolutely no familiarity with the subject matter, with the nature of the theory, with the nature of the experimentation that has been done to eliminate other theories, or with even the most rudimentary basics of cosmology.

As such there's no value in actually debating you on these points.

I would never ask you to debate here. If my points are wrong, show the HN community why. Please exclude any ad-hominem rhetorical maneuvers that are useful to no one. EDIT: re the first sentence: I do not participate here to get value via winning a speaking contest.

This is intellectually dishonest. You're pushing all the work and rigor on others while accepting none for yourself. You're saying "I don't understand this subject, but I still have an opinion, it's your job to also explain the subject to me in addition to explaining why I'm wrong". This is unacceptable in an age where access to the internet (Wikipedia, etc.) makes educating yourself easy. Do better.

I am not sure why you are putting words in my mouth, I never said anything about me not understanding the subject or demanding that you should explain the subject to me.

Please read my post again and if you can, address the points made there.

EDIT: those points, slightly expanded, are:

1) Could it be that Newtonian/Einsteinian models of motion in extremely distant, never visited places are extrapolations in error? If not, how do we know that, since we cannot experiment there, with massive objects on astronomical scales?

2) Is working in line of a popular paradigm much more preferable to most researchers than proposing a radical new theory that is inconsistent with it?

3) Searching for different model / theory is hard work but if it successfuly explains something and provides directions to continue, is preferable to fitting massive quanta of data to a model with infinity degrees of freedom (density of dark matter in space).

4) It is hard to find an example of scientific theory fitting all the evidence. Fitting all the evidence suggests we are doing a fitting exercise, not a scientific theory.

>If my points are wrong

It's your job to not be wrong about knowable things. Read a summary of the literature if you are actually interested instead of just proposing that a well accepted theory is wrong because "theoreticians" are biased.

> It's your job to not be wrong about knowable things.

I was requesting the parent to point out which of my points and why they are wrong. I think reply of such kind would be far more useful to many people here sharing similar views to mine on the idea of dark matter or usefulness of academic work in this direction.

> 'Read a summary of the literature if you are actually interested'

I am actually interested in what people here, including you, think of my points; I was hoping for substantive replies to them. Your suggestion to 'read up and shut up' comes out as arrogant and uncalled for.

> Could it be that our traditional models of motion and light propagation in extremely distant, never visited places are extrapolations in error?

Or whoever programmed our reality couldn't build a consistent macro-physics engine so they hacked something together while hoping biological evolution wouldn't lead to anything that would notice it. Everything was going smoothly with the natural-selection add-on until the platypus and hominids showed up.

No, because of the Bullet Cluster (https://en.wikipedia.org/wiki/Bullet_Cluster).

Dark matter seems to interact weekly even with itself, contrary to regular matter. So it really is completely different stuff, not just matter that's hidden from us.

Because the gravitational pull from these unobservable matter is so huge that little planets, rocks, and dusts cannot account for it.

Sure they can - if there's enough of them. The problem is, "enough of them" should be optically visible too...

All that stuff would emit blackbody radiation if it were regular matter, and we'd be able to see it.

"Dark matter" is a term that means there is a discrepancy between the observed mass in the universe and how much theory says there should be. One possible explanation, and the leading hypothesis, is that there is some form of matter that only interacts gravitionally. Another possibility is that there is something wrong with our current theories. The last one is discussed in the article which is that undetected black holes contain the missing mass.

Yes.

There are four fundamental forces that govern the universe: gravity, electromagnetism, and strong and weak nuclear forces.

Dark matter interacts with gravity, but not electromagnetism. Electromagnetism is the force that we are acting upon when we touch things, or things bump into or collide with one another. Also, it's what makes light and magnetism.

So dark matter would never hit or bump into anything, but it can, say, orbit something.