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> missing most — if not all — of its dark matter Is it just me or does this seem a bit like saying a landline is a wired cell phone? I thought dark matter repr

> missing most — if not all — of its dark matter Is it just me or does this seem a bit like saying a landline is a wired cell phone? I thought dark matter represented what's missing, so if it's not there then nothing is missing, right?

wrs 9y ago
Dark matter is only “missing” in the sense that an explanation of it is missing from established physical theories. It (whatever it is) certainly appears to be present, because it has a major effect on observed galaxy behavior. Otherwise there would be no need to bring up the topic!
- mehrdadn 9y ago
  I get that, it's not that I don't get the reasoning. It's just that it seems backwards to me (like in the phone example). But maybe my comment was an artifact of the fact that I (layman) haven't really bought into the whole idea that we really ever had any "theory" for dark matter in the first place. To me the appropriate headline would've been more like "astronomers find the first galaxy that actually behaves as theories predict".
  - wrs 9y ago
    I don’t think we do have a working theory that explains all the observed evidence. We know the theory without dark matter is wrong, because the vast majority of galaxies don’t agree with it. But we don’t have a new theory that holds together that does explain the observations, with or without dark matter. Basically “dark matter” just means “something that makes the observations work” and the choice of words “dark matter” is because it would work to have some kind of yet-unexplained matter that doesn’t interact with the matter we know about. It looked like it might also work to change the theory of gravity, in which case that was a bad choice of placeholder name — but now this is a counterexample to that idea.
  - ianai 9y ago
    I don’t know the math, but my intuition based guess is it’s related to the range of interaction possible with other particles through QMs probability functions. Ie it’s important to remember that at quantum scale matter/energy transfer through space as probability density waves. Dark matter are at a low bar of the interaction distribution. Something about the empirical values of those equations gives way to many more particles at that low end than expected, but their existence couldn’t ever be ruled out.
teraflop 9y ago
We know that dark matter makes up a large fraction of the universe's overall mass. And we observe dark matter (indirectly) in just about every galaxy we look at. This makes sense, since we believe that galaxies are formed when local concentrations of matter collapse under their own gravity; you'd expect this process to affect both types of matter the same way. When we see a galaxy with no dark matter, it suggests that something unusual is going on, in a way that affects dark matter differently.
- lugg 9y ago
  > We know that dark matter makes up a large fraction of the universe's overall mass. Do we? I thought it was that we know a significant amount of mass is missing from our observations and we explain it with dark matter. Dark matter to me is a crutch to explain things until we really know. Is it not at all possible our understanding is just wrong and that nothing is missing?
  - ianai 9y ago
    Surely all theories are amendable given appropriate data. But we do have a few direct examples of “darkish matter”. Neutrinos definitely exist and pass through the sun faster than their counterparts.
  - Bjartr 9y ago
    Sure, it's just a lot more likely that it's stuff we can't see than our fundamental understanding of a loooot of other things is so incredibly wrong while still being very accurate in practice. I'm sure there are people putting effort into trying to reconcile things in that direction though.
  - tibyat 9y ago
    there are alternate theories of gravity that attempt to explain observed galactic rotation without ‘missing particles’, but they are not as convincing. iirc, the most difficult thing for them to explain is the Bullet Cluster. https://en.m.wikipedia.org/wiki/Bullet_Cluster https://en.m.wikipedia.org/wiki/Bullet_Cluster you’re right in saying this is the first observation that actually fits our gravitational model without missing particles, and that’s why it’s so interesting. it’s much harder to claim your alternate theory of gravity behaves differently around this special galaxy than it is to say there was some event that resulted in the missing particles not being found next to the baryonic matter in the galaxy.
hannasanarion 9y ago
Dark matter isn't "missing". It's invisible. It's something that has gravity but can't be observed. This galaxy is weird because it's mass is equal to the sum of its stars. In every other galaxy, its hundreds or thousands of times more heavy than all the stars put together.

wrs 9y ago

Dark matter is only “missing” in the sense that an explanation of it is missing from established physical theories. It (whatever it is) certainly appears to be present, because it has a major effect on observed galaxy behavior. Otherwise there would be no need to bring up the topic!

mehrdadn 9y ago
I get that, it's not that I don't get the reasoning. It's just that it seems backwards to me (like in the phone example). But maybe my comment was an artifact of the fact that I (layman) haven't really bought into the whole idea that we really ever had any "theory" for dark matter in the first place. To me the appropriate headline would've been more like "astronomers find the first galaxy that actually behaves as theories predict".
- wrs 9y ago
  I don’t think we do have a working theory that explains all the observed evidence. We know the theory without dark matter is wrong, because the vast majority of galaxies don’t agree with it. But we don’t have a new theory that holds together that does explain the observations, with or without dark matter. Basically “dark matter” just means “something that makes the observations work” and the choice of words “dark matter” is because it would work to have some kind of yet-unexplained matter that doesn’t interact with the matter we know about. It looked like it might also work to change the theory of gravity, in which case that was a bad choice of placeholder name — but now this is a counterexample to that idea.
- ianai 9y ago
  I don’t know the math, but my intuition based guess is it’s related to the range of interaction possible with other particles through QMs probability functions. Ie it’s important to remember that at quantum scale matter/energy transfer through space as probability density waves. Dark matter are at a low bar of the interaction distribution. Something about the empirical values of those equations gives way to many more particles at that low end than expected, but their existence couldn’t ever be ruled out.

teraflop 9y ago

We know that dark matter makes up a large fraction of the universe's overall mass. And we observe dark matter (indirectly) in just about every galaxy we look at. This makes sense, since we believe that galaxies are formed when local concentrations of matter collapse under their own gravity; you'd expect this process to affect both types of matter the same way. When we see a galaxy with no dark matter, it suggests that something unusual is going on, in a way that affects dark matter differently.

lugg 9y ago
> We know that dark matter makes up a large fraction of the universe's overall mass. Do we? I thought it was that we know a significant amount of mass is missing from our observations and we explain it with dark matter. Dark matter to me is a crutch to explain things until we really know. Is it not at all possible our understanding is just wrong and that nothing is missing?
- ianai 9y ago
  Surely all theories are amendable given appropriate data. But we do have a few direct examples of “darkish matter”. Neutrinos definitely exist and pass through the sun faster than their counterparts.
- Bjartr 9y ago
  Sure, it's just a lot more likely that it's stuff we can't see than our fundamental understanding of a loooot of other things is so incredibly wrong while still being very accurate in practice. I'm sure there are people putting effort into trying to reconcile things in that direction though.
- tibyat 9y ago
  there are alternate theories of gravity that attempt to explain observed galactic rotation without ‘missing particles’, but they are not as convincing. iirc, the most difficult thing for them to explain is the Bullet Cluster. https://en.m.wikipedia.org/wiki/Bullet_Cluster https://en.m.wikipedia.org/wiki/Bullet_Cluster you’re right in saying this is the first observation that actually fits our gravitational model without missing particles, and that’s why it’s so interesting. it’s much harder to claim your alternate theory of gravity behaves differently around this special galaxy than it is to say there was some event that resulted in the missing particles not being found next to the baryonic matter in the galaxy.

hannasanarion 9y ago

Dark matter isn't "missing". It's invisible. It's something that has gravity but can't be observed. This galaxy is weird because it's mass is equal to the sum of its stars. In every other galaxy, its hundreds or thousands of times more heavy than all the stars put together.