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IANAP. "To detect dark matter, scientists have to wait for the extremely rare occasion when a dark matter particle knocks into an atomic nucleus in the detecto

IANAP. "To detect dark matter, scientists have to wait for the extremely rare occasion when a dark matter particle knocks into an atomic nucleus in the detector and makes it vibrate" Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. It would simply have to be anywhere near us (or a nearby body we could observe carefully) to observe weird gravitational effects. Nearby ones, not 'out there in other galaxies' ones. Assuming it's not near us, why would that be? Our star isn't much affected by it. We can't see nearby stars being pulled willy-nilly. How far out must we search before we could reasonably assume it doesn't exist and there might be another explanation? [edit] And if it's a weird non-localised 'blanket' that can't hit you in the foot, then it'd be great to have some guesses as to the structure so we can test for it. Does it exist inside all galaxies? Is it in the middle, or does it rotate like normal 'matter'? If it exists in the perimeter, is much heavier, and spins like visible matter, do we have to invent a super-ultra-dark matter to explain why common-or-garden dark matter doesn't fly off into space? If it's in the middle and clumpy, could it collapse into a dark-matter black hole, especially since there's no radiation to force particles apart? Is it only one particle type, or a family like visible matter? Since it has mass, is there another form it can convert to as visible mass does? Since it doesn't radiate (being dark) what happens to the energy when (if) it converts? If it's clumpy, stop worrying about visible asteroids. It's the invisible ones that can really hurt you :)

Locke1689 17y ago
Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. Well, if the dark matter is ~90% of the mass of everything, that doesn't necessarily mean that 90% of the mass of every thing is dark matter. As far as I understand it, we see the effects of the large amount of extra matter but we don't really know where it is or how it's distributed. That could mean that there isn't any dark matter clustered in large enough quantities close by for us to detect. Assuming it's not near us, why would that be? Our star isn't much affected by it. We can't see nearby stars being pulled willy-nilly. Well, a really spread out distribution across the universe would explain that. And if it's a weird non-localised 'blanket' that can't hit you in the foot, then it'd be great to have some guesses as to the structure so we can test for it. Well we have a prediction for what the particle would look like (/act like). That's what the detector is there for. Well, not just dark matter. Think neutrinos and Cherenkov radiation. As far as the other questions go, I'm not sure enough to answer and I'm pretty sure that some of those questions are open.
- richardw 17y ago
  I asked mostly for the conversation value, I certainly am not sure of anything myself! Well, a really spread out distribution across the universe would explain that. It would, but then there's the 'why' it's spread out. Maybe that's answered in the particle predictions but something is keeping it apart, but doesn't affect visible matter in the same way. It's obviously spread out just enough to not be clumpy, but not so much that it can't pull a galaxy together. If it were everywhere it wouldn't have the effect we think we see.
  - hugh_ 17y ago
    Ordinary matter forms dense clumps like planets and stars because it's sticky. Atoms may be attracted to one another by gravity, but in order to get them to actually stick together into dust grains, planets, stars et cetera, you need them to collide inelastically, and that requires the electromagnetic force. Two dark-matter particles in the void will be attracted to each other (slightly) by gravity, but when they meet they'll just sail straight on through one another without colliding. This prevents the formation of small-scale (ie sub-galactic scale) dark matter clumps.
andrewcooke 17y ago
> Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. It would simply have to be anywhere near us (or a nearby body we could observe carefully) to observe weird gravitational effects. Nearby ones, not 'out there in other galaxies' ones. two reasons. 1 - dark matter doesn't interact much with normal matter. so even though there's lots of it, most of the time it doesn't actually collide with anything. 2 - the "clumps" of dark matter are at much larger scales than our local system. variations are spread out at something like the size of galaxies; locally it is very smooth / uniform. if you're inside a uniform distribution of mass then you don't feel the gravitational effects (for example, at the centre of the earth you would weigh nothing). disclaimer: i was an astronomer, but have been doing real work for many years now.

Locke1689 17y ago

Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. Well, if the dark matter is ~90% of the mass of everything, that doesn't necessarily mean that 90% of the mass of every thing is dark matter. As far as I understand it, we see the effects of the large amount of extra matter but we don't really know where it is or how it's distributed. That could mean that there isn't any dark matter clustered in large enough quantities close by for us to detect. Assuming it's not near us, why would that be? Our star isn't much affected by it. We can't see nearby stars being pulled willy-nilly. Well, a really spread out distribution across the universe would explain that. And if it's a weird non-localised 'blanket' that can't hit you in the foot, then it'd be great to have some guesses as to the structure so we can test for it. Well we have a prediction for what the particle would look like (/act like). That's what the detector is there for. Well, not just dark matter. Think neutrinos and Cherenkov radiation. As far as the other questions go, I'm not sure enough to answer and I'm pretty sure that some of those questions are open.

richardw 17y ago
I asked mostly for the conversation value, I certainly am not sure of anything myself! Well, a really spread out distribution across the universe would explain that. It would, but then there's the 'why' it's spread out. Maybe that's answered in the particle predictions but something is keeping it apart, but doesn't affect visible matter in the same way. It's obviously spread out just enough to not be clumpy, but not so much that it can't pull a galaxy together. If it were everywhere it wouldn't have the effect we think we see.
- hugh_ 17y ago
  Ordinary matter forms dense clumps like planets and stars because it's sticky. Atoms may be attracted to one another by gravity, but in order to get them to actually stick together into dust grains, planets, stars et cetera, you need them to collide inelastically, and that requires the electromagnetic force. Two dark-matter particles in the void will be attracted to each other (slightly) by gravity, but when they meet they'll just sail straight on through one another without colliding. This prevents the formation of small-scale (ie sub-galactic scale) dark matter clumps.

andrewcooke 17y ago

> Why? If dark matter is 90+% of the mass of 'everything', then surely many more anomalies would occur without us having to be hit by a chunk of dark matter. It would simply have to be anywhere near us (or a nearby body we could observe carefully) to observe weird gravitational effects. Nearby ones, not 'out there in other galaxies' ones. two reasons. 1 - dark matter doesn't interact much with normal matter. so even though there's lots of it, most of the time it doesn't actually collide with anything. 2 - the "clumps" of dark matter are at much larger scales than our local system. variations are spread out at something like the size of galaxies; locally it is very smooth / uniform. if you're inside a uniform distribution of mass then you don't feel the gravitational effects (for example, at the centre of the earth you would weigh nothing). disclaimer: i was an astronomer, but have been doing real work for many years now.