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So the way I understand it dark matter doesn't even have any physical interaction with matter. But it must be affected by gravity, or else it wouldn't end up tr

So the way I understand it dark matter doesn't even have any physical interaction with matter. But it must be affected by gravity, or else it wouldn't end up trapped in black holes right?

qubex 9y ago
Well, the very reason for conceiving of dark matter is because there is evidence of gravitational effects from mass we cannot observe. So dark matter by definition must at minimum interact gravitationally, and be hard to observe by other means (electromagnetically, maybe even intent to the nuclear forces). Black holes sans accretion disk clearly satisfy these criteria.
notzorbo3 9y ago
As far as I understand it: All kinds of stuff is happening in the universe that can only happen if there's much more gravity than we can explain with the matter we can actually observe. So there has to be something more than the observable matter. But we can't find it, so the assumption was that it didn't physically interact like normal matter (except for gravity), otherwise it would give at least some indication that it's there. Like infrared or something, I dunno. But if it turns out it's black holes, I'm guessing it would have physical interaction with other matter but still be hard to detect and not interact in the usual way we expect of normal matter. All this is speculation though, and I don't really know what I'm talking about.
- ryandrake 9y ago
  Ignorant "ELI5" question from a non-astrophysics major: Couldn't all this unobservable matter be just normal but so-far unobservable matter? Little planets and rocks all over the place that we just can't see yet because our telescopes aren't good enough? Why do we think it has to be some other "exotic" form of matter?
  - ww520 9y ago
    Because the gravitational pull from these unobservable matter is so huge that little planets, rocks, and dusts cannot account for it.
    - AnimalMuppet 9y ago
      Sure they can - if there's enough of them. The problem is, "enough of them" should be optically visible too...
  - notzorbo3 9y ago
    Well, I'm really not qualified to answer, but I think it's because regardless of how small each individual piece of "debris" would be, together it should still form a huge amount of matter. All matter gives of electromagnetic radiation. If there was normal matter there, we'd see the electromagnetic radiation, but we don't. So it has to be something else. It's kinda like how we can see those epic cloud nebula formations. Those clouds are basically just dust and gas and other tiny stuff. And it's not as clumped together as you'd think, but we can still detect it lightyears away.
    - pducks32 9y ago
      There are other more complicated reasons but this is a pretty good answer. Basically that matter at some point must have been at the Big Bang and with all the heat generated around that time some of it must have interacted with the debris that GP is talking about. We don’t see evidence of that. It’s important to remember that though we as humans lose things all the time. The universe cannot. We got what we got.
  - bbojan 9y ago
    No, because of the Bullet Cluster (https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster). Dark matter seems to interact weekly even with itself, contrary to regular matter. So it really is completely different stuff, not just matter that's hidden from us.
  - InclinedPlane 9y ago
    No, we've tested for that repeatedly. That was, of course, the first thing scientists thought of to explain this excess non-luminous mass. Could it be large transparent gas clouds? No, it turns out that gas clouds aren't completely transparent, and can be measured in various ways. We looked, we checked, such clouds don't contribute enough mass. Could it be dust clouds? Dust clouds are not luminous but they do block visible light, so they'd be obvious. We looked, we checked. It wasn't dust clouds. Could it be rogue planets or brown dwarfs? No, such objects would cause gravitational micro-lensing effects, which can be observed in dedicated campaigns. We looked, we checked. It wasn't planets or brown dwarfs. The only theory that has withstood the last several decades of experimental observation and simulation has been the dark matter theory (weakly interacting massive particles at sub-relativistic speeds). It's the only thing that explains all the evidence across the board.
    - effie 9y ago
      Could it be that our traditional models of motion and light propagation in extremely distant, never visited places are extrapolations in error? It seems most astronomy theoreticians will rather not rock the boat, they will rather keep believing in invisible stuff permeating space everywhere with variable but exactly right density so it is consistent with star motions and images observed. I guess it's because stuff with infinity of degrees of freedom is traditionally much more secure paradigm to base a career on than to question the established beliefs in applicability of Newtonian mechanics/General relativity. > "The only theory that has withstood the last several decades of experimental observation and simulation" Which competing theories were put to stand the test of observation? What made them not stand well in light of observations? > "It's the only thing that explains all the evidence" There is little example to such a thing as scientific theory explaining all the evidence. But even if dark matter paradigm did do such a feat, it would not be a good sign for the truthfulness of it. Theories that explain everything are suspicious - they may be exercises in fitting and may not be so scientific or useful as competing theories that explain less, but better.
      - icanhackit 9y ago
        > Could it be that our traditional models of motion and light propagation in extremely distant, never visited places are extrapolations in error? Or whoever programmed our reality couldn't build a consistent macro-physics engine so they hacked something together while hoping biological evolution wouldn't lead to anything that would notice it. Everything was going smoothly with the natural-selection add-on until the platypus and hominids showed up.
  - Retra 9y ago
    All that stuff would emit blackbody radiation if it were regular matter, and we'd be able to see it.
ww520 9y ago
Dark matter does interaction with matter via gravity. It just doesn't interact via electromagnetism, the weak nuclear force, or the strong nuclear force. Our usual observation techniques with electromagnetism can't detect it.
- 37 9y ago
  Yes. In fact, this is how we detect DM, by measuring the speeds of the galaxies, rotating around the galactic core. Our calculations point to a slower speed than observed, thus implying some mass of "matter" which we cannot see.
lawpoop 9y ago
Yes. There are four fundamental forces that govern the universe: gravity, electromagnetism, and strong and weak nuclear forces. Dark matter interacts with gravity, but not electromagnetism. Electromagnetism is the force that we are acting upon when we touch things, or things bump into or collide with one another. Also, it's what makes light and magnetism. So dark matter would never hit or bump into anything, but it can, say, orbit something.
rpedela 9y ago
"Dark matter" is a term that means there is a discrepancy between the observed mass in the universe and how much theory says there should be. One possible explanation, and the leading hypothesis, is that there is some form of matter that only interacts gravitionally. Another possibility is that there is something wrong with our current theories. The last one is discussed in the article which is that undetected black holes contain the missing mass.

So the way I understand it dark matter doesn't even have any physical interaction with matter. But it must be affected by gravity, or else it wouldn't end up trapped in black holes right?

qubex 9y ago

Well, the very reason for conceiving of dark matter is because there is evidence of gravitational effects from mass we cannot observe. So dark matter by definition must at minimum interact gravitationally, and be hard to observe by other means (electromagnetically, maybe even intent to the nuclear forces). Black holes sans accretion disk clearly satisfy these criteria.

notzorbo3 9y ago

As far as I understand it: All kinds of stuff is happening in the universe that can only happen if there's much more gravity than we can explain with the matter we can actually observe. So there has to be something more than the observable matter. But we can't find it, so the assumption was that it didn't physically interact like normal matter (except for gravity), otherwise it would give at least some indication that it's there. Like infrared or something, I dunno. But if it turns out it's black holes, I'm guessing it would have physical interaction with other matter but still be hard to detect and not interact in the usual way we expect of normal matter. All this is speculation though, and I don't really know what I'm talking about.

ryandrake 9y ago
Ignorant "ELI5" question from a non-astrophysics major: Couldn't all this unobservable matter be just normal but so-far unobservable matter? Little planets and rocks all over the place that we just can't see yet because our telescopes aren't good enough? Why do we think it has to be some other "exotic" form of matter?
- ww520 9y ago
  Because the gravitational pull from these unobservable matter is so huge that little planets, rocks, and dusts cannot account for it.
  - AnimalMuppet 9y ago
    Sure they can - if there's enough of them. The problem is, "enough of them" should be optically visible too...
- notzorbo3 9y ago
  Well, I'm really not qualified to answer, but I think it's because regardless of how small each individual piece of "debris" would be, together it should still form a huge amount of matter. All matter gives of electromagnetic radiation. If there was normal matter there, we'd see the electromagnetic radiation, but we don't. So it has to be something else. It's kinda like how we can see those epic cloud nebula formations. Those clouds are basically just dust and gas and other tiny stuff. And it's not as clumped together as you'd think, but we can still detect it lightyears away.
  - pducks32 9y ago
    There are other more complicated reasons but this is a pretty good answer. Basically that matter at some point must have been at the Big Bang and with all the heat generated around that time some of it must have interacted with the debris that GP is talking about. We don’t see evidence of that. It’s important to remember that though we as humans lose things all the time. The universe cannot. We got what we got.
- bbojan 9y ago
  No, because of the Bullet Cluster (https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster). Dark matter seems to interact weekly even with itself, contrary to regular matter. So it really is completely different stuff, not just matter that's hidden from us.
- InclinedPlane 9y ago
  No, we've tested for that repeatedly. That was, of course, the first thing scientists thought of to explain this excess non-luminous mass. Could it be large transparent gas clouds? No, it turns out that gas clouds aren't completely transparent, and can be measured in various ways. We looked, we checked, such clouds don't contribute enough mass. Could it be dust clouds? Dust clouds are not luminous but they do block visible light, so they'd be obvious. We looked, we checked. It wasn't dust clouds. Could it be rogue planets or brown dwarfs? No, such objects would cause gravitational micro-lensing effects, which can be observed in dedicated campaigns. We looked, we checked. It wasn't planets or brown dwarfs. The only theory that has withstood the last several decades of experimental observation and simulation has been the dark matter theory (weakly interacting massive particles at sub-relativistic speeds). It's the only thing that explains all the evidence across the board.
  - effie 9y ago
    Could it be that our traditional models of motion and light propagation in extremely distant, never visited places are extrapolations in error? It seems most astronomy theoreticians will rather not rock the boat, they will rather keep believing in invisible stuff permeating space everywhere with variable but exactly right density so it is consistent with star motions and images observed. I guess it's because stuff with infinity of degrees of freedom is traditionally much more secure paradigm to base a career on than to question the established beliefs in applicability of Newtonian mechanics/General relativity. > "The only theory that has withstood the last several decades of experimental observation and simulation" Which competing theories were put to stand the test of observation? What made them not stand well in light of observations? > "It's the only thing that explains all the evidence" There is little example to such a thing as scientific theory explaining all the evidence. But even if dark matter paradigm did do such a feat, it would not be a good sign for the truthfulness of it. Theories that explain everything are suspicious - they may be exercises in fitting and may not be so scientific or useful as competing theories that explain less, but better.
    - icanhackit 9y ago
      > Could it be that our traditional models of motion and light propagation in extremely distant, never visited places are extrapolations in error? Or whoever programmed our reality couldn't build a consistent macro-physics engine so they hacked something together while hoping biological evolution wouldn't lead to anything that would notice it. Everything was going smoothly with the natural-selection add-on until the platypus and hominids showed up.
- Retra 9y ago
  All that stuff would emit blackbody radiation if it were regular matter, and we'd be able to see it.

ww520 9y ago

Dark matter does interaction with matter via gravity. It just doesn't interact via electromagnetism, the weak nuclear force, or the strong nuclear force. Our usual observation techniques with electromagnetism can't detect it.

37 9y ago
Yes. In fact, this is how we detect DM, by measuring the speeds of the galaxies, rotating around the galactic core. Our calculations point to a slower speed than observed, thus implying some mass of "matter" which we cannot see.

lawpoop 9y ago

Yes. There are four fundamental forces that govern the universe: gravity, electromagnetism, and strong and weak nuclear forces. Dark matter interacts with gravity, but not electromagnetism. Electromagnetism is the force that we are acting upon when we touch things, or things bump into or collide with one another. Also, it's what makes light and magnetism. So dark matter would never hit or bump into anything, but it can, say, orbit something.

rpedela 9y ago

"Dark matter" is a term that means there is a discrepancy between the observed mass in the universe and how much theory says there should be. One possible explanation, and the leading hypothesis, is that there is some form of matter that only interacts gravitionally. Another possibility is that there is something wrong with our current theories. The last one is discussed in the article which is that undetected black holes contain the missing mass.