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When I see dark matter in the news I'm always reminded of the story of Vulcan. In the 1800s, detailed observations of the planet Mercury showed that its orbit

When I see dark matter in the news I'm always reminded of the story of Vulcan. In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors. Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun. Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790 https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed. When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects. Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete? MOND may not be the solution, but I'm still skeptical about dark matter.

gwynforthewyn 5mo ago
Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis. Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.
- MattPalmer1086 5mo ago
  As far as I know, dark matter hasn't made any testable predictions which were subsequently proved correct. It's an infinitely tunable hypothesis that can be retro fitted to new observations, since it has many tweakable parameters. Could you point me to a prediction made by dark matter theory which was subsequently confirmed by observation, rather than a dark matter theory which explains it post facto?
  - MattPalmer1086 5mo ago
    Not sure why the downvotes just for asking a question. Dark matter (well, LCDM) has predicted many things which were proven false, e.g. that big galaxies assembled through merging with smaller ones over long periods of time. JWST shows us this is not in fact the case. There are now lots of papers showing how LCDM can explain that, but these are post facto explanations. Happy to be proven wrong if someone can point me to a prediction subsequently confirmed by observation.
    - gus_massa 5mo ago
      Is this the "hot" vs "cold" dark mater discussion? (Not my area. I just read that in Wikipedia.)
      - MattPalmer1086 4mo ago
        Not really, Cold Dark Matter is part of LCDM, the main cosmological model. Whether hot or cold, I'd like to see a solid prediction made for it confirmed subsequently by observation.
Centigonal 5mo ago
yes - absent experimental confirmation, the effects attributed to dark matter may in fact be due to an unrelated as-yet-undiscovered phenomenon.
pessimist 5mo ago
There are multiple independent observations pointing toward dark matter: 1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg). MOND only explains 1. Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
- deleted 5mo ago
  [deleted]
- dnautics 5mo ago
  MOND explains 1. 2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND). 3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g. MOND explains several things LCDM cannot: - why most elliptical galaxies seem to "not have dark matter" (effectively a prediction) - external field effect (predicted and confirmed) - renzo's rule - DM halos that are way too big - early galaxies (this was a prediction) HM. people have been downvoting. Anyone care to post a substantive rebuttal?
  - lukan 5mo ago
    "HM. people have been downvoting. Anyone care to post a substantive rebuttal?" Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)
    - dnautics 5mo ago
      I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.
lukeify 5mo ago
The constituent particles of nature are under no requirement to be apparent or achievable to you.
lubujackson 5mo ago
100%. It's important to realize our understanding of "dark matter" is fuzzy because we only understand it through data anomalies. Dark matter is a classic catch-all concept that we use as a crutch while we try to understand the underlying system better. Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.
- kstrauser 5mo ago
  It's kind of weird in this case, though. All the math acts like there's something invisible and heavy everywhere that we find clumps of visible matter. When we look at the motion of galaxies, they behave as if they're much more massive than the count of stars and such in the would have you believe, and in ways that otherwise jibe with our understanding of physics if only that galaxy were heavier. If you have one galaxy that's acting heavier than you can eyeball, measured by things like light bending around it, then maybe you have some weird phenomenon. When every galaxy calculates out to be about 6x fatter than you'd expect, something else is going on.
  - fragmede 5mo ago
    Except (IANotAnAstrophysicist) it's not every galaxy. There are two that stick out as having little to no dark matter. * NGC 1052-DF2 * NGC 1052-DF4 Of course this is hotly contested as it destroys the argument for MOND, but research is ongoing.
gostsamo 5mo ago
You might be interested in that. We have observations of galaxies where dark matter likely is not present. https://news.ycombinator.com/item?id=47641057 https://news.ycombinator.com/item?id=47641057
not2b 5mo ago
The strongest evidence that something like MOND isn't the answer is that in some galaxy collisions, the visible matter and the dark matter appear to separate: the collision disrupts the visible matter and the dark matter appears to pass right through, uninterrupted, and we see galaxy remnants that look like they don't have dark matter. If MOND or some other modification of gravity were the answer we'd never see this kind of sorting. See, for example, https://www.caltech.edu/about/news/dark-matter-flies-ahead-of-normal-matter-in-mega-galaxy-cluster-collision https://www.caltech.edu/about/news/dark-matter-flies-ahead-o...
rcxdude 5mo ago
Astrophysicists have gone all through this. Absent direct observation or at least a good explanation of what it actually is, I don't think anyone really likes dark matter. The main objection is that it feels a little too flexible: you can explain a lot of things by adding mass in different places. But it's not infinitely flexible: you don't need arbitrary and utterly weird distributions of dark matter. And so it's generally emerged as the best explanation of observations so far, and not for lack of trying with tweaks to theories of gravity themselves.

gwynforthewyn 5mo ago

Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis. Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.

MattPalmer1086 5mo ago
As far as I know, dark matter hasn't made any testable predictions which were subsequently proved correct. It's an infinitely tunable hypothesis that can be retro fitted to new observations, since it has many tweakable parameters. Could you point me to a prediction made by dark matter theory which was subsequently confirmed by observation, rather than a dark matter theory which explains it post facto?
- MattPalmer1086 5mo ago
  Not sure why the downvotes just for asking a question. Dark matter (well, LCDM) has predicted many things which were proven false, e.g. that big galaxies assembled through merging with smaller ones over long periods of time. JWST shows us this is not in fact the case. There are now lots of papers showing how LCDM can explain that, but these are post facto explanations. Happy to be proven wrong if someone can point me to a prediction subsequently confirmed by observation.
  - gus_massa 5mo ago
    Is this the "hot" vs "cold" dark mater discussion? (Not my area. I just read that in Wikipedia.)
    - MattPalmer1086 4mo ago
      Not really, Cold Dark Matter is part of LCDM, the main cosmological model. Whether hot or cold, I'd like to see a solid prediction made for it confirmed subsequently by observation.

Centigonal 5mo ago

yes - absent experimental confirmation, the effects attributed to dark matter may in fact be due to an unrelated as-yet-undiscovered phenomenon.

pessimist 5mo ago

There are multiple independent observations pointing toward dark matter: 1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg). MOND only explains 1. Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.

deleted 5mo ago
[deleted]
dnautics 5mo ago
MOND explains 1. 2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND). 3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g. MOND explains several things LCDM cannot: - why most elliptical galaxies seem to "not have dark matter" (effectively a prediction) - external field effect (predicted and confirmed) - renzo's rule - DM halos that are way too big - early galaxies (this was a prediction) HM. people have been downvoting. Anyone care to post a substantive rebuttal?
- lukan 5mo ago
  "HM. people have been downvoting. Anyone care to post a substantive rebuttal?" Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)
  - dnautics 5mo ago
    I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.

lukeify 5mo ago

The constituent particles of nature are under no requirement to be apparent or achievable to you.

lubujackson 5mo ago

100%. It's important to realize our understanding of "dark matter" is fuzzy because we only understand it through data anomalies. Dark matter is a classic catch-all concept that we use as a crutch while we try to understand the underlying system better. Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.

kstrauser 5mo ago
It's kind of weird in this case, though. All the math acts like there's something invisible and heavy everywhere that we find clumps of visible matter. When we look at the motion of galaxies, they behave as if they're much more massive than the count of stars and such in the would have you believe, and in ways that otherwise jibe with our understanding of physics if only that galaxy were heavier. If you have one galaxy that's acting heavier than you can eyeball, measured by things like light bending around it, then maybe you have some weird phenomenon. When every galaxy calculates out to be about 6x fatter than you'd expect, something else is going on.
- fragmede 5mo ago
  Except (IANotAnAstrophysicist) it's not every galaxy. There are two that stick out as having little to no dark matter. * NGC 1052-DF2 * NGC 1052-DF4 Of course this is hotly contested as it destroys the argument for MOND, but research is ongoing.

gostsamo 5mo ago

You might be interested in that. We have observations of galaxies where dark matter likely is not present. https://news.ycombinator.com/item?id=47641057 https://news.ycombinator.com/item?id=47641057

not2b 5mo ago

The strongest evidence that something like MOND isn't the answer is that in some galaxy collisions, the visible matter and the dark matter appear to separate: the collision disrupts the visible matter and the dark matter appears to pass right through, uninterrupted, and we see galaxy remnants that look like they don't have dark matter. If MOND or some other modification of gravity were the answer we'd never see this kind of sorting. See, for example, https://www.caltech.edu/about/news/dark-matter-flies-ahead-of-normal-matter-in-mega-galaxy-cluster-collision https://www.caltech.edu/about/news/dark-matter-flies-ahead-o...

rcxdude 5mo ago

Astrophysicists have gone all through this. Absent direct observation or at least a good explanation of what it actually is, I don't think anyone really likes dark matter. The main objection is that it feels a little too flexible: you can explain a lot of things by adding mass in different places. But it's not infinitely flexible: you don't need arbitrary and utterly weird distributions of dark matter. And so it's generally emerged as the best explanation of observations so far, and not for lack of trying with tweaks to theories of gravity themselves.