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Comment by amluto | Hacker News Reader

Not quite.

No one knows what dark matter is. Astrophysicists know it by how it behaves. It seems to behave just like a normal gas, or perhaps dust or even moderate chunky stuff, except that it appears to be essentially completely transparent and simply passes through normal matter.

(There’s one candidate theory of dark matter called, IIRC, “MACHO”s, which is the idea that dark matter is just a bunch of small, very dense objects dotting the universe. We don’t see them because they’re compact and don’t take up enough space to be noticed.)

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How would we possibly know it passes through other matter?

I believe it's also reasoned to be a consequence of the apparent non-interaction with the electromagnetic force that dark matter has. Since it doesn't block light in any way we can detect (absorption, changes in polarization, changes in direction, etc.) it's also not likely to actually "touch" normal matter in the way you'd intuitively expect. That's because that touch is mediated via the electromagnetic force, the electron clouds and the charges they have repelling each other over the vast empty spaces of normal matter.

That’s one reason. As another example, a lot of the dust and gas in our solar system ended up in the sun, presumably because it collided with other stuff that formed the sun, thus losing speed and getting trapped. But there isn’t a bunch of dark matter in the sun. So either there wasn’t a bunch of diffuse dark matter in the stuff that made up the early solar system or the dark matter was somehow immune from getting trapped along with all the normal matter.

Strictly speaking we don't, but we have reasons to believe that on a galactic scale it doesn't. There was that famous example a few years back where we had observed two galaxies hitting each other, and it "looked" like the regular ol' matter (eg, interstellar gas clouds) in each galaxy had "hit" each other and stopped while the dark matter in each had passed through the other galaxy unimpeded.

Which would seem to indicate that either dark matter is sparse and heavy (and just doesn't hit things very often) or diffuse and not-very-interacting with other matter.

I am not convinced. What can possibly be said about two galaxies "hitting" each other if we observe only a sliver of time? Secondly, the stopping would happen because of gravity, and that would affect dark matter too.

Gas clouds also interact electromagnetically: https://en.wikipedia.org/wiki/Bullet_Cluster

You shouldn't be convinced by one sentence saying that a conclusion was reached a few years back. But you also have no basis for confidence in the negation of that conclusion on the basis of that almost total lack of data or theory.

You can't honestly believe that teams of people with physics Ph.D.s just didn't address these questions - how do we infer from observational data, what do we infer from that data - so go read the paper.

I think they mean we only observe other matter interacting with other matter, and not dark matter interacting with other matter.

No, dark matter does interact with other matter: but only through gravitational forces, not electromagnetic or nuclear ones.

What would interaction of nuclear forces predict on such huge scales?

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