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There is no direct evidence for dark matter. It is a placeholder for observations that don’t match theory. There are many current experiments trying to detect p

There is no direct evidence for dark matter. It is a placeholder for observations that don’t match theory. There are many current experiments trying to detect proposed particles but so far none has been detected.

DiogenesKynikos 7y ago
It's much more than a placeholder for observations that don't match theory. There are a whole bunch of very different observations that don't match theory in different ways, if your theory doesn't include dark matter. But if your theory includes dark matter, all those various experiments match theory pretty darn perfectly. There's no a priori reason why something like dark matter shouldn't exist. It would be nice to directly detect it through scattering experiments on Earth, but there are a lot of possibilities for what dark matter is, and each would require a different type of experiment to detect.
- davrosthedalek 7y ago
  Worst case, all couplings to the SM might be so small that it's undetectable on earth. A purely gravitational direct detection is probably out of reach for quite a while.
  - DiogenesKynikos 7y ago
    That would be pretty disappointing, but as far as I understand it, entirely possible. There are three ways to detect dark matter: 1. scattering in a lab 2. annihilation products observed with a telescope 3. gravitational effects We've seen 3, and we'll probably get a huge amount more detail on the distribution of dark matter through that method as time goes on. That would already be pretty convincing to me. People have been looking for both 1 and 2 for a while, without any conclusive detection. But it's easy to write down theories of dark matter that are extremely difficult to detect with those methods. At some point, we may have to accept that the only way to detect dark matter is method 3, through gravitational effects.
    - davrosthedalek 7y ago
      That's why I think it is so important to do precision tests of the Standard model here on earth. DM might show up as differences between theory and measurement. There are a couple of puzzles like that, g_muon-2, proton radius, ^8Be,... Maybe we get lucky.
scarejunba 7y ago
Well, it's a placeholder, but a very specific kind of placeholder: it fits exactly everywhere. It's not like it's special-cased to one of the problems. It actually works in all of them.
- LoSboccacc 7y ago
  yeah but you'd get consistent results whether is actual mass or a wrong value somewhere in a fundamental constant, the fact that the dark matter proportions comes out always in the same ballpark, alone, tells us not very much.
- denton-scratch 7y ago
  The reason it "fits exactly everywhere" is that the only definition we have is that it's the exactly gap between observation and theory.
  - DiogenesKynikos 7y ago
    No, the definition is cold, gravitationally interacting matter. It just so happens that adding that one extra ingredient reconciles a whole number of different observations with theory. That didn't have to be the case. For example, if the dark matter is hot, structure formation doesn't match observations, or if dark matter is composed of massive, compact objects, we would observe a lot more microlensing events. But putting in just the right amount of cold dark matter makes everything we can test so far work. That could be a giant coincidence, but most cosmologists think it's because cold dark matter really exists.

DiogenesKynikos 7y ago

It's much more than a placeholder for observations that don't match theory. There are a whole bunch of very different observations that don't match theory in different ways, if your theory doesn't include dark matter. But if your theory includes dark matter, all those various experiments match theory pretty darn perfectly. There's no a priori reason why something like dark matter shouldn't exist. It would be nice to directly detect it through scattering experiments on Earth, but there are a lot of possibilities for what dark matter is, and each would require a different type of experiment to detect.

davrosthedalek 7y ago
Worst case, all couplings to the SM might be so small that it's undetectable on earth. A purely gravitational direct detection is probably out of reach for quite a while.
- DiogenesKynikos 7y ago
  That would be pretty disappointing, but as far as I understand it, entirely possible. There are three ways to detect dark matter: 1. scattering in a lab 2. annihilation products observed with a telescope 3. gravitational effects We've seen 3, and we'll probably get a huge amount more detail on the distribution of dark matter through that method as time goes on. That would already be pretty convincing to me. People have been looking for both 1 and 2 for a while, without any conclusive detection. But it's easy to write down theories of dark matter that are extremely difficult to detect with those methods. At some point, we may have to accept that the only way to detect dark matter is method 3, through gravitational effects.
  - davrosthedalek 7y ago
    That's why I think it is so important to do precision tests of the Standard model here on earth. DM might show up as differences between theory and measurement. There are a couple of puzzles like that, g_muon-2, proton radius, ^8Be,... Maybe we get lucky.

scarejunba 7y ago

Well, it's a placeholder, but a very specific kind of placeholder: it fits exactly everywhere. It's not like it's special-cased to one of the problems. It actually works in all of them.

LoSboccacc 7y ago
yeah but you'd get consistent results whether is actual mass or a wrong value somewhere in a fundamental constant, the fact that the dark matter proportions comes out always in the same ballpark, alone, tells us not very much.
denton-scratch 7y ago
The reason it "fits exactly everywhere" is that the only definition we have is that it's the exactly gap between observation and theory.
- DiogenesKynikos 7y ago
  No, the definition is cold, gravitationally interacting matter. It just so happens that adding that one extra ingredient reconciles a whole number of different observations with theory. That didn't have to be the case. For example, if the dark matter is hot, structure formation doesn't match observations, or if dark matter is composed of massive, compact objects, we would observe a lot more microlensing events. But putting in just the right amount of cold dark matter makes everything we can test so far work. That could be a giant coincidence, but most cosmologists think it's because cold dark matter really exists.