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You’ve also neglected to mention the evidence that the articles are leaving out ;0

Filligree 6y ago
For example, galaxy collisions. Dark matter by its nature doesn't collide, so it behaves differently than the galaxies it's in when there are physical collisions, and that's visible. Basically, the galaxies stop; the dark matter keeps going. We can't directly see it, but we can see the gravitational lensing it causes. Dark matter isn't as invisible as believed. Then there are cases of galaxies which don't contain any dark matter, and therefore act the way we were expecting them to from simple gravitational mechanics on visible matter. That rules out any solution involving changes to the laws of gravity, since such a change would be universal. The fact that galaxies can exist which contain none means it must be a substance of some kind which can exist more in one spot than another.
- lostmsu 6y ago
  Why does the dark matter stop? Don't colliding galaxies stop due to gravity, which dark matter is equally affected by?
  - yongjik 6y ago
    Galaxies are drawn toward each other due to gravity, but when they meet ordinary matters "stop" because lumps of gas plow against each other, causing them to lose energy and stop. Dark matter doesn't interact with anything else (or even itself) so they pass right through each other.
    - chadcmulligan 6y ago
      So does that mean there are some galaxies without dark matter - because of a previous collision? And then some dark matter only galaxies, because they left their galaxy behind? I suppose this is unknown - but does the dark matter reappear in the galaxy later somehow, or it's left behind forever? Edit: well there are galaxies without dark matter (maybe) [1] and there are dark matter galaxies [2] [1] https://www.space.com/19-galaxies-missing-dark-matter.html https://www.space.com/19-galaxies-missing-dark-matter.html [2] https://www.space.com/33850-weird-galaxy-is-mostly-dark-matter.html https://www.space.com/33850-weird-galaxy-is-mostly-dark-matt...
      - corty 6y ago
        reappearance is possible in collision sims, but the timescale is too long to be easily observable.
      - The_rationalist 6y ago
        I remember that a few years ago a galaxy had been discovered with a claimed ~>90% of dark matter. Personally I believe the theory is epistemologically weaker than MOND
  - nwallin 6y ago
    Same reason cars stop when they hit a brick wall. Most of the mass of galaxies isn't in their stars (which, for the most part, keep moving with the dark matter) it's in gas clouds and dust formations. When two galaxies collide, the gas and dust effectively halts. What we see when galaxies collide is that most of the mass of the galaxies move straight through each other without appreciably decelerating. And most of the matter, the gas clouds and star forming regions, halt in the middle. MOND was taken seriously as a theory from its inception in the '70s through the end of the 20th century, but because of the bullet cluster, it is effectively dead. MOND is falsified by observational evidence. For this article to discuss MOND so thoroughly, but not mention the bullet cluster once... It doesn't pass the smell test. This is pseudo-scientific trash. There's other evidence against MOND and various alternative-gravity models (there are several of them, it's a large umbrella) but most of them do a very poor job at explaining the edge cases, such as the bullet cluster and ultra diffuse galaxies. Some ultra diffuse galaxies have a conspicuous absence of gravitational lensing and stars which orbit extraordinarily slowly; ie, they have very little dark matter. If the explanation for dark matter was that we had the laws of physics wrong, the newly revised gravitational theories should apply equally well to these galaxies- which they don't.
    - manquer 6y ago
      While the article is perhaps not written as well it could have been, alternative gravity is probably indeed bust, the point that other theories need to be funded and studied is valid. it is not a either/or scenario. The argument the article makes is that just because emergent gravity or many alternative candidates potentially cannot explain all of evidence that dark matter can , does not mean it should be thrown out completely and rejected, it could be still funded and studied to see if it can explain some of it. The real issue is given the way modern academic funding works we have been tending more and more towards narrower research with main line theories and lesser riskier proposals perhaps being incentivised. This is not just limited not dark matter , this phenomenon is evident in string theory research and many other fields as well.
      - narrator 6y ago
        How about we fund somebody like a George Church to just a have a lab and pursue whatever he feels like in cosmology instead of such a narrow "school of thought" picked by a donor?
  - pdonis 6y ago
    > Why does the dark matter stop? The dark matter doesn't stop; it keeps going. > Don't colliding galaxies stop due to gravity Not really. As others have noted, they stop due to non-gravitational interactions between the ordinary matter (mainly gas and dust clouds).
- thegabriele 6y ago
  "Then there are cases of galaxies which don't contain any dark matter..." You mean zero at ALL, or with little measurable effects?
  - occamrazor 6y ago
    We don’t know, more likely the latter. We can see that some galaxies rotate as if they had no dark matter, but we don’t know why their dark matter isn’t there.The prevalent, but still speculative, hypothesis is that it was “stolen” by a bigger galaxy in a close pass-by.
    - enkid 6y ago
      So does dark matter interact with itself outside of gravity? If not, how does the visible matter separate from dark matter.
      - antepodius 6y ago
        If you imagine that dark matter and visible matter both start out basically smoothly distributed in a big cloud, they'll both tend to collapse inwards. The matter forms a rotating disk because the particles of matter interact through non-gravitational means; so ones that get close can exchange (and cancel!) momentum. But if dark matter only interacts gravitationally, it won't do this; the particles will just fall through the center of the future galaxy, 'passing through' each other (and the matter). Gravitational drag presumably adds some rotation to the dark matter over time; I don't know. But that's how dark matter and regular matter could separate. >So does dark matter interact with itself outside of gravity? There are different ideas about what dark matter could be. Some have it interacting through other forces (WIMPs interacted through the weak force, for example), some are purely gravitational. There's always talk of 'dark matter candidates', theoretical or otherwise particles that could plausibly be dark matter.
knzhou 6y ago
- gravitational lensing measurements - velocities in galaxy clusters - getting the fine details of the complicated, wiggly cosmic microwave background spectrum right to extreme precision - getting the fine details of cosmological structure formation (as measured from matter density fluctuations on very large scales) right - galaxy collisions, like the Bullet cluster (hard to explain with modified gravity) - the fact that some galaxies have much less dark matter than others (very hard to explain with modified gravity)
- rocqua 6y ago
  For the last point. Where do we get our confidence in the (light) mass estimation of a galaxy? Couldn't we be off by a factor of 10?
  - knzhou 6y ago
    Mostly the confidence comes from half a century of trying to find such "extra" visible mass! There's a long history of people looking for extra dust or gas or compact objects using a tremendous variety of clever techniques, and yet the deficit has remained. At this point, if you believe that there indeed is extra mass around, it kind of has to be dark matter by definition because it's been so damn hard to see.
    - NohatCoder 6y ago
      The question is how the f do we know the mass of a galaxy in the first place? We can't have the discrepancy until we have two distinct methods of measuring the mass of a galaxy.
astroH 6y ago
The best evidence for dark matter is arguably the CMB. The strengths of the individual peaks are highly suggestive of dark matter, in particular the third peak. Furthermore, most MOND simulations on cosmological scales still require dark matter (if they evolve the Universe following the Friedmann equation), for example an 11eV sterile neutrino, because there has yet to be an underlying theory that accurately describes the evolution that can reproduce the mass function and galaxy clustering with MOND. This isn't to say that dark matter doesn't have its problem, one of which is the fact that we don't know what it is; however, there still remain numerous candidates that have yet to be sufficiently explored experimentally.
- prox 6y ago
  Is there a scenario where both could be right in some way, so part due to DM and part due to modified gravity?
  - astroH 6y ago
    Modified gravity is still being actively explored and I would argue taken very seriously but not in the context of dark matter, rather in the context of dark energy. So in a sense, yes, modified and dark matter combined are both possible in the same theory. However, at the moment, there is no real reason to suggest both dark matter and modified gravity are responsible for things like galaxy rotation curves because one of these works just as well without the other. In fact, many modified gravity theories apply "screening mechanisms" so they don't impact galaxy scales
  - kwbauson 6y ago
    Sabine Hossenfelder has a great series of videos about both dark matter and modified gravity theories (https://www.youtube.com/watch?v=FN2d2cmi_Gk https://www.youtube.com/watch?v=FN2d2cmi_Gk, https://www.youtube.com/watch?v=2VNcDoLNJk8 https://www.youtube.com/watch?v=2VNcDoLNJk8, and https://www.youtube.com/watch?v=468cyBZ_cq4 https://www.youtube.com/watch?v=468cyBZ_cq4). I think she does a very thorough (and relatively easy to understand) job of explaining what both of the types of theories are, the motivations for developing them, and their strengths and weaknesses. The first video is about dark matter, the second is about modified gravity (mostly focusing on MOND), and in the third she talks about how she thinks it makes sense to accept both. Even if you don't agree with her position, her videos are still very informative for anybody who wants to know more.

For example, galaxy collisions. Dark matter by its nature doesn't collide, so it behaves differently than the galaxies it's in when there are physical collisions, and that's visible. Basically, the galaxies stop; the dark matter keeps going. We can't directly see it, but we can see the gravitational lensing it causes. Dark matter isn't as invisible as believed. Then there are cases of galaxies which don't contain any dark matter, and therefore act the way we were expecting them to from simple gravitational mechanics on visible matter. That rules out any solution involving changes to the laws of gravity, since such a change would be universal. The fact that galaxies can exist which contain none means it must be a substance of some kind which can exist more in one spot than another.

lostmsu 6y ago
Why does the dark matter stop? Don't colliding galaxies stop due to gravity, which dark matter is equally affected by?
- yongjik 6y ago
  Galaxies are drawn toward each other due to gravity, but when they meet ordinary matters "stop" because lumps of gas plow against each other, causing them to lose energy and stop. Dark matter doesn't interact with anything else (or even itself) so they pass right through each other.
  - chadcmulligan 6y ago
    So does that mean there are some galaxies without dark matter - because of a previous collision? And then some dark matter only galaxies, because they left their galaxy behind? I suppose this is unknown - but does the dark matter reappear in the galaxy later somehow, or it's left behind forever? Edit: well there are galaxies without dark matter (maybe) [1] and there are dark matter galaxies [2] [1] https://www.space.com/19-galaxies-missing-dark-matter.html https://www.space.com/19-galaxies-missing-dark-matter.html [2] https://www.space.com/33850-weird-galaxy-is-mostly-dark-matter.html https://www.space.com/33850-weird-galaxy-is-mostly-dark-matt...
    - corty 6y ago
      reappearance is possible in collision sims, but the timescale is too long to be easily observable.
    - The_rationalist 6y ago
      I remember that a few years ago a galaxy had been discovered with a claimed ~>90% of dark matter. Personally I believe the theory is epistemologically weaker than MOND
- nwallin 6y ago
  Same reason cars stop when they hit a brick wall. Most of the mass of galaxies isn't in their stars (which, for the most part, keep moving with the dark matter) it's in gas clouds and dust formations. When two galaxies collide, the gas and dust effectively halts. What we see when galaxies collide is that most of the mass of the galaxies move straight through each other without appreciably decelerating. And most of the matter, the gas clouds and star forming regions, halt in the middle. MOND was taken seriously as a theory from its inception in the '70s through the end of the 20th century, but because of the bullet cluster, it is effectively dead. MOND is falsified by observational evidence. For this article to discuss MOND so thoroughly, but not mention the bullet cluster once... It doesn't pass the smell test. This is pseudo-scientific trash. There's other evidence against MOND and various alternative-gravity models (there are several of them, it's a large umbrella) but most of them do a very poor job at explaining the edge cases, such as the bullet cluster and ultra diffuse galaxies. Some ultra diffuse galaxies have a conspicuous absence of gravitational lensing and stars which orbit extraordinarily slowly; ie, they have very little dark matter. If the explanation for dark matter was that we had the laws of physics wrong, the newly revised gravitational theories should apply equally well to these galaxies- which they don't.
  - manquer 6y ago
    While the article is perhaps not written as well it could have been, alternative gravity is probably indeed bust, the point that other theories need to be funded and studied is valid. it is not a either/or scenario. The argument the article makes is that just because emergent gravity or many alternative candidates potentially cannot explain all of evidence that dark matter can , does not mean it should be thrown out completely and rejected, it could be still funded and studied to see if it can explain some of it. The real issue is given the way modern academic funding works we have been tending more and more towards narrower research with main line theories and lesser riskier proposals perhaps being incentivised. This is not just limited not dark matter , this phenomenon is evident in string theory research and many other fields as well.
    - narrator 6y ago
      How about we fund somebody like a George Church to just a have a lab and pursue whatever he feels like in cosmology instead of such a narrow "school of thought" picked by a donor?
- pdonis 6y ago
  > Why does the dark matter stop? The dark matter doesn't stop; it keeps going. > Don't colliding galaxies stop due to gravity Not really. As others have noted, they stop due to non-gravitational interactions between the ordinary matter (mainly gas and dust clouds).
thegabriele 6y ago
"Then there are cases of galaxies which don't contain any dark matter..." You mean zero at ALL, or with little measurable effects?
- occamrazor 6y ago
  We don’t know, more likely the latter. We can see that some galaxies rotate as if they had no dark matter, but we don’t know why their dark matter isn’t there.The prevalent, but still speculative, hypothesis is that it was “stolen” by a bigger galaxy in a close pass-by.
  - enkid 6y ago
    So does dark matter interact with itself outside of gravity? If not, how does the visible matter separate from dark matter.
    - antepodius 6y ago
      If you imagine that dark matter and visible matter both start out basically smoothly distributed in a big cloud, they'll both tend to collapse inwards. The matter forms a rotating disk because the particles of matter interact through non-gravitational means; so ones that get close can exchange (and cancel!) momentum. But if dark matter only interacts gravitationally, it won't do this; the particles will just fall through the center of the future galaxy, 'passing through' each other (and the matter). Gravitational drag presumably adds some rotation to the dark matter over time; I don't know. But that's how dark matter and regular matter could separate. >So does dark matter interact with itself outside of gravity? There are different ideas about what dark matter could be. Some have it interacting through other forces (WIMPs interacted through the weak force, for example), some are purely gravitational. There's always talk of 'dark matter candidates', theoretical or otherwise particles that could plausibly be dark matter.

knzhou 6y ago

- gravitational lensing measurements - velocities in galaxy clusters - getting the fine details of the complicated, wiggly cosmic microwave background spectrum right to extreme precision - getting the fine details of cosmological structure formation (as measured from matter density fluctuations on very large scales) right - galaxy collisions, like the Bullet cluster (hard to explain with modified gravity) - the fact that some galaxies have much less dark matter than others (very hard to explain with modified gravity)

rocqua 6y ago
For the last point. Where do we get our confidence in the (light) mass estimation of a galaxy? Couldn't we be off by a factor of 10?
- knzhou 6y ago
  Mostly the confidence comes from half a century of trying to find such "extra" visible mass! There's a long history of people looking for extra dust or gas or compact objects using a tremendous variety of clever techniques, and yet the deficit has remained. At this point, if you believe that there indeed is extra mass around, it kind of has to be dark matter by definition because it's been so damn hard to see.
  - NohatCoder 6y ago
    The question is how the f do we know the mass of a galaxy in the first place? We can't have the discrepancy until we have two distinct methods of measuring the mass of a galaxy.

astroH 6y ago

The best evidence for dark matter is arguably the CMB. The strengths of the individual peaks are highly suggestive of dark matter, in particular the third peak. Furthermore, most MOND simulations on cosmological scales still require dark matter (if they evolve the Universe following the Friedmann equation), for example an 11eV sterile neutrino, because there has yet to be an underlying theory that accurately describes the evolution that can reproduce the mass function and galaxy clustering with MOND. This isn't to say that dark matter doesn't have its problem, one of which is the fact that we don't know what it is; however, there still remain numerous candidates that have yet to be sufficiently explored experimentally.

prox 6y ago
Is there a scenario where both could be right in some way, so part due to DM and part due to modified gravity?
- astroH 6y ago
  Modified gravity is still being actively explored and I would argue taken very seriously but not in the context of dark matter, rather in the context of dark energy. So in a sense, yes, modified and dark matter combined are both possible in the same theory. However, at the moment, there is no real reason to suggest both dark matter and modified gravity are responsible for things like galaxy rotation curves because one of these works just as well without the other. In fact, many modified gravity theories apply "screening mechanisms" so they don't impact galaxy scales
- kwbauson 6y ago
  Sabine Hossenfelder has a great series of videos about both dark matter and modified gravity theories (https://www.youtube.com/watch?v=FN2d2cmi_Gk https://www.youtube.com/watch?v=FN2d2cmi_Gk, https://www.youtube.com/watch?v=2VNcDoLNJk8 https://www.youtube.com/watch?v=2VNcDoLNJk8, and https://www.youtube.com/watch?v=468cyBZ_cq4 https://www.youtube.com/watch?v=468cyBZ_cq4). I think she does a very thorough (and relatively easy to understand) job of explaining what both of the types of theories are, the motivations for developing them, and their strengths and weaknesses. The first video is about dark matter, the second is about modified gravity (mostly focusing on MOND), and in the third she talks about how she thinks it makes sense to accept both. Even if you don't agree with her position, her videos are still very informative for anybody who wants to know more.