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Comment by erikpukinskis | Hacker News Reader

I’m a lay person, but...

Isn’t dark matter just matter in other dimensions? Not visible in our three, but obviously having an effect? Like the body of an arch to flatlanders?

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Not sure why an honest question is getting downvotes.

1) We do not yet have any reason to believe there are more than 3 dimensions plus time [1] 2) It depends on what you mean by "visible". Take a look at this picture full screen: https://en.wikipedia.org/wiki/Dark_matter#/media/File:Gravit...

That is an image of dark matter bending the light from distant galaxies - take note of the circularly shaped bent galaxies radiating around the middle. This effect is called "gravitational lensing", and it is why we know that there actually is some matter in this picture. We also know that it is invisible because otherwise the "dark matter" would be blocking view of the galaxies you're looking at.

3) I highly recommend reading the summary on the Wikipedia article on dark matter [2]. The whole article is really long, but the summary paragraphs at the top give a great high-level explanation of "what we know" about dark matter.

[1] https://physics.stackexchange.com/questions/4079/experimenta... [2] https://en.wikipedia.org/wiki/Dark_matter

Humans have a long history of misunderstanding the geometry of our world. I don't know jack about astrophysics, but the explanations for dark matter and dark energy seem so complicated that my bullshit meter goes off. Reminds me of the people trying to model the walk of the planets in an Earth-centric universe. I'd put money on another Newton or Einstein (or singularity AI) giving us a much more elegant solution in the next 50-100 years.

I feel similarly dissatisfied with the explanations but, given the current framework, they're actually the simplest possible models that could work. The rest of physics and the many observations that need to be consistent with it are just that elaborate.

Unfortunately, there's not enough hard evidence to support the claim that dark matter resides in other dimensions. All we know for sure is that, based on our current understanding of physics, the observable universe behaves as if it has more mass than what we are capable of detecting.

You might want to check out a very interesting non-linear spinor theory[1] which contains a chapter on dark matter and dark energy analytically explaining the formation and inner structure, as well as the ratio of dark matter and dark energy throughout the development phase of the universe.

[1] http://norbert-winter.com/wp-content/uploads/2018/02/2017-03...

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.)

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?

I guess the options aere: (1) dark matters is a property of physics. IE, it is the effects of physics that we don't understand yet, but has consequences to matter and the properties/rules of matter that we do understand. (2) Dark matter is stuff, actual matter that we just can't "see".

Matter in other dimensions is I guess a combination of the two. Actual normal matter, "hidden" by properties of physics that we don't quite understand.

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