It would be a lot easier to detect if it did, but let me defer to CERN.
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It was only through observations of issues like the galaxy rotation problem and other theories that it was even noticed.
But basically, it it interacted with EM radiation we could simply see it.
Explain it to me like I’m really stupid. I understand that if a large object were absorbing light we’d detect it because it would be black. But what about small objects?
It may comprise four-fifths of the mass in the universe of all the matter in the universe, even if it was small we would see as a fog, or a cloud even if that interaction was pretty weak.
In theory, millions and possibly billions of particles of dark matter are passing through every square inch of your body each hour, if it interacted with the EM field we would notice.
Handwaving mightily because this is well past the edge of my reading, much less my even more limited understanding:
Is that best guess to say that in some sense the entire hole over its entire history is a quantum superposition? Such that everything becomes entangled to everything else everytime else?
But unlike a “conventional” particle superposition, where observing one particle collapses the wavefunction of the other(s), this seems like we’d need to see everything about everything everytime to know anything about anything anytime? IOW, the logical inverse of a superposition.
Or am I just babbling here? 
I think you’re just babbling there. Unless you can say it in math, it’s safest to not say anything about quantum mechanics at all-- Even the top minds in the field don’t really have an intuition for it.
OK, fair enough. Thank you.
I’ve given essentially the same admonition to abashed in his pi & circle threads: “intuition safely applies to things you understand well, not to things you know you don’t. Overextended intuition is just guesswork masquerading as confidence.”
Since it was pointed out to me that I’d missed this:
If it were in tiny clumps, you’d need a lot of them, and they’d form a sort of gray fog across the sky. If instead they were in large clumps (called “MACHOs”, for "MAssive Compact Halo Objects), they’d be much harder to notice, at least via their overall fogging effect. But you could still detect them through what’s called microlensing: Every so often, one such clump would pass between us and a star, and its gravitational field would slightly bend light around it, causing a focusing effect which would make the star appear to momentarily brighten in a particular way. Microlensing searches have been done, and have concluded that MACHOs can account for only a small fraction of the dark matter.
But there’s another problem with MACHOs: What are they made of? We have very good bounds on the total amount of baryons (protons and neutrons) in the Universe, based on the relative abundances of isotopes of the lightweight elements. And that’s also far short of the dark matter total. Since our familiar sort of matter gets almost all of its mass from protons and neutrons, there must be some other sort of matter making up the dark matter. And if that nonbaryonic dark matter stuff, whatever it is, interacts with the electromagnetic force, then why can’t we detect any of it in our particle-accelerator experiments?
BTW, Google suggests some physicists have proposed black holes as dark matter. Could these be considered light absorbing"?
Isn’t there some intermediate size (in ballpark of solar mass, perhaps?) in which a compact object would be hard to detect either way? Whether via fog or lens?
I Googled the question at stackexchange and found similar appeals to models, or lack thereof, rather than presenting any direct evidence that “dark matter does not absorb light.” For example:
I wonder if some arguments against light-absorbing dark matter reduce to an assumption (via 2nd Law of ThermoDynamics?) that material which absorbs light will also emit it.
Might this confidence evaporate in some baryonic models of dark matter?
Our models of Big Bang nucleosynthesis are quite strong, and hence our confidence in the amount of baryonic matter is quite high. No dark matter model is anywhere near strong enough to significantly shake that confidence.
Black holes might be some or all of the dark matter, but then you have to ask where they came from. The baryonic-matter limits still apply, so either the holes would have to have been formed from something other than baryonic matter, or they would have to have been formed before the era of nucleosynthesis, very early in the Universe’s history. And if primordial, low-mass black holes are really so abundant, then you also have to ask why we’ve never detected any of them. This is a problem in general with the more exotic hypotheses for dark matter: The more exotic something is, the more likely it should be that we would have detected it in some other way.
And the microlensing MACHO searches are good at least as far down as planetary masses (and would also, incidentally, detect black holes in that mass range). I don’t know how large dark-matter objects could be without producing the fog effect.
Put it all together, and we don’t know that the bulk of the dark matter is made up of stuff that doesn’t interact electromagnetically at all, but it seems by far the safest way to bet. It’s really not so surprising, after all, that there would be some such particles, and if they exist, there would be a good reason why we don’t easily detect them.
Thank you, Chronos. This is a different and more developed argument than anything I saw at stackexchange. Especially interesting is the fact that physicists know how many baryons in total the universe holds. Can you point me to a webpage where I can read more about this?
Well, here’s the Wikipedia page. I’m not sure what other online resources would be any good; I learned about this stuff from textbooks.
I should also point out that not all of these possibilities are actually completely ruled out. Picture a game of Battleship, where you’re trying to find that pesky little two-peg gunboat. There’s one region of your board which you’ve covered almost completely with white pegs, with no reds and just a few gaps left. And there’s another region of the board where you’ve taken almost no shots at all. Now, the gunboat might be in one of those gaps in the first region, but if someone asked you to bet, which region do you think it’s in?
I’d quibble there’s a tiny bit of circular reasoning there:
How do we know our nucleosynthesis models are good? Because they match what we measure, except for dark matter. But we don’t expect dark matter to be baryonic, so that’s okay.
Why don’t we expect dark matter to be baryonic? Because it doesn’t match our nucleosynthesis models. And our nucleosynthesis models are good.
This isn’t really meant to be criticism. A tiny bit of circular reasoning is another way of saying “self consistent”. That’s a good thing. We just have to careful not to completely foreclose other faint possibilities.
Your analogy here is a good one:
Well, the real key with the Big Bang nucleosynthesis results is that the data we have is overdetermined: Just knowing the relative abundances for two or three isotopes would be enough to calculate the baryon abundance, but we’ve got something like six or seven isotopes to work with, and they all agree. If it were just one or two, it’d be easy to say “well, maybe there’s something we’re not considering that changes the ratios”, but with that many all matching, it’s tough to come up with any other explanation that matches all of them.