[ziont]

what is dark matter? is it the opposite of matter? how can it be called matter when it doesn't exist?

or are these objects from higher dimensions as Michio Kaku postulates? that galaxies are essentially in a bubble of dark matter preventing it from hurling in every direction.

as well as the presence of dark matter in orbits, could it not be the space-time curvature from the mass itself but from the encapsulating dark matter around it?

I no very little of this subject so I can only ponder and fantasize.

[krapp]

Via Wikipedia[0]

    Dark matter is a hypothetical form of matter that is thought 
    to account for approximately 85% of the matter in the universe, 
    and about a quarter of its total energy density. 

    (...)

    Its presence is implied in a variety of astrophysical 
    observations, including gravitational effects that 
    cannot be explained unless more matter is present 
    than can be seen. 

Also, since it will inevitably come up as it always does in threads about dark matter or dark energy , here is an article[1] and related[2] HN thread arguing the theoretical justifications behind the existence of dark matter, what is known or can be inferred about its properties and why chances are no one missed whatever simple solution you can think of, like "maybe it's just normal matter but they can't see it because it's far away" or "maybe it's just black holes" and why it's also not entirely hubris or ignorance on the part of the scientific community.

[0]https://en.wikipedia.org/wiki/Dark_matter

[1]https://medium.com/starts-with-a-bang/this-is-the-real-reaso...

[2]https://news.ycombinator.com/item?id=18207276

[rjf72]

The article you link there does not mention the SISSA discovery of 2016 [1]. In particular in mini-spiral galaxies there seems to be an unexpected, and inexplicable, interaction between dark and visible matter. This falsifies the primary prediction put forth in that medium article. And in general this issue is one that poses a critical problem for dark matter largely because it cannot just be massaged into the model.

And that last point is really the problem with model based physics. They are really really hard to falsify which can provide certainty through inertia. For instance take the heliocentric vs geocentric earth. The geocentric view (that the Earth was the center upon which everything rotated) was based upon a model. It started out fairly simple and intuitive. But then we kept coming upon ever more issues. For instance if everything else rotates around Earth then that must mean other planets travel in sort of 'swirly' type patterns that we see nowhere else in physics. Well, okay - why not. Hmm it also turns out that some planets, such as Mercury, need to just magically stop in their orbit and start going backwards at some point. Again, it's a model that we can't really falsify so okay - why not.

And it absorbs really bizarre observation after another. The only way to refute it was ultimately to be able to see 'through the eyes of god' that geocentricism was wrong. This was not hubris or ignorance on part of the scientific community. It was inertia. Hundreds of years scientific study was built upon the assumption of a geocentric Earth. Astrology was a scholarly pursuit, at least as reputable as psychology is today, developed over centuries. And it was 100% dependent upon the geocentric model. We've rewritten history to blame geocentricism on the church alone, but there was far more in play there. Going against the geocentric model meant having the arrogance to call numerous well reputed fields completely wrong, to completely discredit and repudiate the work of the most brilliant minds of the times, and to basically say you somehow know better than hundreds of years of work and pretty much everybody else.

No, it's certainly not hubris that drives inertia. It's the lack thereof. Einstein's theory of relativity sounds absolutely insane. It took the sort of man who would go on to condescendingly mock quantum entanglement as "spooky action at a distance" to have the sort of ego and self confidence, bordering on hubris, to be able to not only consider such possibilities but to spend years of countless effort refining and developing it. Much of this done when he, unable to find a professorial position, was working in a patent office - no less! Indeed if we have any bias in play, I'd expect it's rather a contagion of humility driven by the exponential growth in complexity.

If anything, the arrogance would come in the form of an aggressive disregard for alternatives. See the practically militant response to things such as MOND [2] hypotheses. It still suffers many problems, as does dark matter, but remains a viable alternative hypothesis which remains rather unexplored. Of course there is a practical issue there. Since nobody expects things such as MOND to be correct, which it probably is not, then spending years proving that is effectively wasted. By contrast as most people expect dark matter to be correct, proving it right or wrong would be an achievement worthy of a trip to Stockholm. It means pursuing MOND (or other alternatives) is a very difficult choice, in terms of career progression and prospects. Scientific inertia is a problem with no clear solution.

[1] - https://phys.org/news/2016-12-unexpected-interaction-dark-or...

[2] - https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

[magicalhippo]

> what is dark matter? is it the opposite of matter? how can it be called matter when it doesn't exist?

Dark matter is what we call the discrepancy between our observations of how galaxies and galaxy clusters move, and how our theories tell us they should move based on our best estimates for how much normal matter is in those galaxies.

We call it "dark matter" because our formulas give the right prediction if we add a lot of mass where we see none, that is, if we assume there is some matter there that does not emit or interact with light.

AFAIK, the difference between general relativity with 3 spatial dimensions and one with additional spatial dimensions show up on the small scale, and not large scale like galaxies[1].

> could it not be the space-time curvature from the mass itself but from the encapsulating dark matter around it

If dark matter is indeed some kind of matter, then it would indeed affect the space-time curvature. That's in fact the entire point behind calling it dark _matter_, as mentioned above.

Of course there is the possibility that our theories needs to change. There have been proposed alternatives that does not require additional matter in the form of new particles. The problem is that it's very difficult to get the theories to match non-dark matter observations, all the "normal stuff", as well as dark matter observations.

[1]: https://www.forbes.com/sites/briankoberlein/2016/04/26/looki...

[clubm8]

>We call it "dark matter" because our formulas give the right prediction if we add a lot of mass where we see none, that is, if we assume there is some matter there that does not emit or interact with light.

I'm not a physics expert so forgive me if this is a naive take, but could black holes just sitting around, too far from other objects to have a visible effect, account for this unaccounted mass? IIRC they're small in size but incredibly heavy, and if space is infinite then they could just be sitting around taking up mass.

[magicalhippo]

Someone has thought of this, mainly involving primordial black holes. IIRC the idea is that right after the big bang, when the subtle density differences in the gas permeating the entire universe caused pockets of gas to contract and eventually form stars, some pockets would collapse into black holes directly.

edit: I should add that they can't be regular black holes formed from stars, because then they'd have the same distribution of the stars in the galaxy they formed. While the the stars in a galaxy are most numerous near the center and get less numerous away from the center, the mass we have to insert in the form of dark matter to make the equations add up has to be evenly spread from the center and far beyond the visible edge of the galaxy.

The problem with this primordial black hole idea is that having lots of such black holes about would leave a very tell-tale signature thanks to gravitational lensing, which would be very strong near the event horizon. So far, no searches have found this.

Here's one news article describing the results of a recent search: https://physicsworld.com/a/supernovae-reveal-that-primordial...

See also https://en.wikipedia.org/wiki/Primordial_black_hole

[clubm8]

It makes me feel good I came up with a similar theory to what a trained physicist did, even if it's likely wrong...

Thanks for the links, I'm on a wikipedia binge now :)

[magicalhippo]

You're welcome!

I think part of the issue with dark matter and similar is that the main stream mostly hears about the theories that are the best candidates, and not about all the alternative explanations physicists have thought about but discarded due to some more or less obvious issues. Most won't even make it to arXiv, since they'll be eliminated by some rudimentary cross checks.

So the public might get the impression that physicists don't even try to come up with alternatives, but that couldn't be further from the truth.

[PurpleRamen]

Dark matter is the hole in the formulas, the dark spot were no lights shines yet. We don't really know whether it exists or not, we only know that our models of the universe have this strange corner were things don't work as they should work. And because all the others corners work so well, we assume that there must be something else.

Its called dark matter because the strange corner is in the models regarding matter. Thre is also dark energy, a strange corner in our models of energy.