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Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are s

Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are stretched between galaxies. Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.

colechristensen 9y ago
"dark matter" is a blanket term for stuff we have had a hard time seeing but we very strongly believe is there Some of the candidates for where all the missing mass is, is poorly interacting matter
throwaway613834 9y ago
> basically only gravity and not EM or other forces I don't understand how physicists make any sense of this in any kind of theory. If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? How does that work? Or would dark matter just somehow resist even interacting with itself to create friction, etc.?
- justin_vanw 9y ago
  Friction, and really any kind of 'contact' between things is just electrons repelling. Some ideas for dark matter don't interact in any way other than gravity, so your idea is pretty mainstream on that, but there's basically no evidence that dark matter exists in some specific form, and in fact every experiment seems to constrain the possible forms it could have more and more. It's still possible it exists, but I would suspect that whatever the explanation is for the 'missing' matter will be pretty revolutionary.
- nikdaheratik 9y ago
  We don't know much about what it does do, only what it doesn't do. Scientists have deduced its existence by measuring the speed of the expansion of the universe and determining that the mass that we can see based on the rotation of galaxies, etc. can not account for the speed of the expansion. There should be way more mass out there than there is. We also know that it doesn't seem to react with light otherwise it would block out the stars from other galaxies as well as the cosmic background radiation. It's difficult to tell how it reacts with ordinary matter because we can't see it, but the assumption is that, since its got such a large gravitational effect, it must not react much at all otherwise it would dominate everything visible since it's 90% of the matter in the known universe. It's difficult to make sense of what this means, but all of the other theories that explain the rotation of galaxies and the expansion of the universe don't fit very well either.
  - Analog24 9y ago
    Just a clarification: galaxy rotation curves and the observed expansion of the universe are not really related. The empirically observed expansion of the Universe is the motivating evidence for the existence of Dark Energy. Dark Matter, which is motivated (in part) by the inconsistencies in the rotation curves of galaxies is a separate topic. Despite sharing similarities in their names and the fact that they make up the two biggest chunks of energy/mass in the Universe, Dark Energy and Dark Matter are not actually related. The word 'dark' is really just implying that we have not yet observed anything to explain these two phenomenon.
- dragonwriter 9y ago
  > If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? You can't. Because becoming a star (initiating nuclear fusion) requires nongravitational interaction between nucleons, which are normal, not dark, matter.
  - throwaway613834 9y ago
    I thought the claim was that dark matter doesn't interact with normal matter through other forces, not that it doesn't interact with itself? But if so... then dark matter might be just bosons? The properties I'm reading seem to be pretty consistent with bosons.
    - dragonwriter 9y ago
      IIUC, If it interacts with itself through other forces, they can't be through the forces that act on normal matter (which they would interact with if that was the case), including the forces involved in nuclear fusion. They'd have to be dark matter exclusive forces.
      - throwaway613834 9y ago
        Interesting, thanks. So what's preventing "dark matter" from simply being, say, lots of photons traveling through intergalactic space then? That seems like the next obvious candidate after ordinary matter.
        gnaritas 9y ago
        Photons lack mass right?
        posterboy 9y ago
        not exactly. As photons are constantly moving at the speed of light (citing the theory of relativity I don't really understand) they have no resting mass. But they have an impuls and create a force of impact (used in sun sails) from which a theoretical notion of ... mass (kinetic, perhaps?) can be derived. I'd like to know how it's derived originally, too though. edit: if E=mc² with E(photon)=f•h and f=c/λ, then m=h/(λc) in vacuum. ... I hope that's correct. What's interesting, [m]=J/[a]/m
- InclinedPlane 9y ago
  A star begins as a huge lump of gravitationally bound gas. This gas runs into each other, that's how it has pressure and temperature, even when it is as diffuse as a proto-stellar nebula is (which can be on the order of tens to hundreds of atoms per cubic centimeter). The nebula goes through cycles of compression and radiation, as the gas collapses due to gravitation it heats up, raising the pressure and halting the collapse. But then the heat is radiated away and the gas cools, and then it continues to collapse. This process takes a chunk of gas that is on the order of a light-year across and follows it down and down and down as it contracts to the size of a solar system then to the size of a star. Over time the globule gets denser and denser, and thus the force of gravity pulling matter down toward the center gets stronger and stronger. Which means that the amount of pressure necessary to counteract that pull gets higher and higher, and thus the temperature of the proto-star as it collapses goes up. Stars are born hot and bright, even before conditions in their core are hot enough to ignite fusion reactions. Which is the inevitable result of a mass of gas that is dense enough to undergo collapse and is massive enough to result in a body that is over about 75 times the mass of Jupiter (the lower limit of a red dwarf star). It is the energy from those fusion reactions which provide the temperature and pressure increases which ultimately halt further gravitational collapse. That's how stars are formed. Absolutely none of this is applicable to dark matter. Dark matter isn't made up of atoms, it doesn't bump into and bounce off of other particles of matter. It doesn't maintain a temperature and pressure the way a gas does. Dark matter interacts extremely weakly. Neutrinos are an example of dark matter, but a type that we know doesn't make up most of the mass of dark matter in the Universe. A neutrino will pass through a chunk of lead a light year thick and then only have a 50/50 chance of being stopped. Dark matter is even more weakly interacting (with ordinary matter and itself). Particles of dark matter are zooming about in orbits around the center of mass of our galaxy. They zip through almost everything they touch without interacting, the exception being black holes, which they simply fall into like everything else, of course. They are like an enormous parade of ghosts that can only interact with other matter through gravitation, meaning orbital dynamics. Because of this they have no way of condensing into forms of higher density like nebulae, stars, or planets. Imagine a giant ball of yarn larger than our galaxy, except each thread is a flow, a river of huge numbers of ghostly dark matter particles. Except there are many balls of yarn overlayed on top of one another and many flows going through any one point, since they don't interact with each other. Some are traveling around in orbits around the Milky Way in the same direction as our Solar System, some are going the opposite way, some are in orbits at various inclinations to the galactic plane, some are in circular orbits, some are in eccentric orbits and the part of the galaxy where we are is their highest distance from the galactic center, for others it's the closest distance to the galactic center, and so on. All of these flows, this ghostly wind of insubstantial but massive dark matter particles is what together makes up the "dark matter halo" around our galaxy and around typical galaxies. In any given section of the galaxy, say a typical cubic light year, the total amount of dark matter isn't that great, it's vastly lower than the mass of any star that would happen to be there, for example. But the dark matter is everywhere, it flows throughout a region that extends well beyond the edge of the visible galaxy, and it has roughly the same density everywhere, so over huge volumes that mass adds up, and up, and up, and turns out to be, in aggregate, greater than the mass of all of the ordinary matter in our galaxy, by a factor of about 5:1.

colechristensen 9y ago

"dark matter" is a blanket term for stuff we have had a hard time seeing but we very strongly believe is there Some of the candidates for where all the missing mass is, is poorly interacting matter

throwaway613834 9y ago

> basically only gravity and not EM or other forces I don't understand how physicists make any sense of this in any kind of theory. If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? How does that work? Or would dark matter just somehow resist even interacting with itself to create friction, etc.?

justin_vanw 9y ago
Friction, and really any kind of 'contact' between things is just electrons repelling. Some ideas for dark matter don't interact in any way other than gravity, so your idea is pretty mainstream on that, but there's basically no evidence that dark matter exists in some specific form, and in fact every experiment seems to constrain the possible forms it could have more and more. It's still possible it exists, but I would suspect that whatever the explanation is for the 'missing' matter will be pretty revolutionary.
nikdaheratik 9y ago
We don't know much about what it does do, only what it doesn't do. Scientists have deduced its existence by measuring the speed of the expansion of the universe and determining that the mass that we can see based on the rotation of galaxies, etc. can not account for the speed of the expansion. There should be way more mass out there than there is. We also know that it doesn't seem to react with light otherwise it would block out the stars from other galaxies as well as the cosmic background radiation. It's difficult to tell how it reacts with ordinary matter because we can't see it, but the assumption is that, since its got such a large gravitational effect, it must not react much at all otherwise it would dominate everything visible since it's 90% of the matter in the known universe. It's difficult to make sense of what this means, but all of the other theories that explain the rotation of galaxies and the expansion of the universe don't fit very well either.
- Analog24 9y ago
  Just a clarification: galaxy rotation curves and the observed expansion of the universe are not really related. The empirically observed expansion of the Universe is the motivating evidence for the existence of Dark Energy. Dark Matter, which is motivated (in part) by the inconsistencies in the rotation curves of galaxies is a separate topic. Despite sharing similarities in their names and the fact that they make up the two biggest chunks of energy/mass in the Universe, Dark Energy and Dark Matter are not actually related. The word 'dark' is really just implying that we have not yet observed anything to explain these two phenomenon.
dragonwriter 9y ago
> If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? You can't. Because becoming a star (initiating nuclear fusion) requires nongravitational interaction between nucleons, which are normal, not dark, matter.
- throwaway613834 9y ago
  I thought the claim was that dark matter doesn't interact with normal matter through other forces, not that it doesn't interact with itself? But if so... then dark matter might be just bosons? The properties I'm reading seem to be pretty consistent with bosons.
  - dragonwriter 9y ago
    IIUC, If it interacts with itself through other forces, they can't be through the forces that act on normal matter (which they would interact with if that was the case), including the forces involved in nuclear fusion. They'd have to be dark matter exclusive forces.
    - throwaway613834 9y ago
      Interesting, thanks. So what's preventing "dark matter" from simply being, say, lots of photons traveling through intergalactic space then? That seems like the next obvious candidate after ordinary matter.
      - gnaritas 9y ago
        Photons lack mass right?
        posterboy 9y ago
        not exactly. As photons are constantly moving at the speed of light (citing the theory of relativity I don't really understand) they have no resting mass. But they have an impuls and create a force of impact (used in sun sails) from which a theoretical notion of ... mass (kinetic, perhaps?) can be derived. I'd like to know how it's derived originally, too though. edit: if E=mc² with E(photon)=f•h and f=c/λ, then m=h/(λc) in vacuum. ... I hope that's correct. What's interesting, [m]=J/[a]/m
InclinedPlane 9y ago
A star begins as a huge lump of gravitationally bound gas. This gas runs into each other, that's how it has pressure and temperature, even when it is as diffuse as a proto-stellar nebula is (which can be on the order of tens to hundreds of atoms per cubic centimeter). The nebula goes through cycles of compression and radiation, as the gas collapses due to gravitation it heats up, raising the pressure and halting the collapse. But then the heat is radiated away and the gas cools, and then it continues to collapse. This process takes a chunk of gas that is on the order of a light-year across and follows it down and down and down as it contracts to the size of a solar system then to the size of a star. Over time the globule gets denser and denser, and thus the force of gravity pulling matter down toward the center gets stronger and stronger. Which means that the amount of pressure necessary to counteract that pull gets higher and higher, and thus the temperature of the proto-star as it collapses goes up. Stars are born hot and bright, even before conditions in their core are hot enough to ignite fusion reactions. Which is the inevitable result of a mass of gas that is dense enough to undergo collapse and is massive enough to result in a body that is over about 75 times the mass of Jupiter (the lower limit of a red dwarf star). It is the energy from those fusion reactions which provide the temperature and pressure increases which ultimately halt further gravitational collapse. That's how stars are formed. Absolutely none of this is applicable to dark matter. Dark matter isn't made up of atoms, it doesn't bump into and bounce off of other particles of matter. It doesn't maintain a temperature and pressure the way a gas does. Dark matter interacts extremely weakly. Neutrinos are an example of dark matter, but a type that we know doesn't make up most of the mass of dark matter in the Universe. A neutrino will pass through a chunk of lead a light year thick and then only have a 50/50 chance of being stopped. Dark matter is even more weakly interacting (with ordinary matter and itself). Particles of dark matter are zooming about in orbits around the center of mass of our galaxy. They zip through almost everything they touch without interacting, the exception being black holes, which they simply fall into like everything else, of course. They are like an enormous parade of ghosts that can only interact with other matter through gravitation, meaning orbital dynamics. Because of this they have no way of condensing into forms of higher density like nebulae, stars, or planets. Imagine a giant ball of yarn larger than our galaxy, except each thread is a flow, a river of huge numbers of ghostly dark matter particles. Except there are many balls of yarn overlayed on top of one another and many flows going through any one point, since they don't interact with each other. Some are traveling around in orbits around the Milky Way in the same direction as our Solar System, some are going the opposite way, some are in orbits at various inclinations to the galactic plane, some are in circular orbits, some are in eccentric orbits and the part of the galaxy where we are is their highest distance from the galactic center, for others it's the closest distance to the galactic center, and so on. All of these flows, this ghostly wind of insubstantial but massive dark matter particles is what together makes up the "dark matter halo" around our galaxy and around typical galaxies. In any given section of the galaxy, say a typical cubic light year, the total amount of dark matter isn't that great, it's vastly lower than the mass of any star that would happen to be there, for example. But the dark matter is everywhere, it flows throughout a region that extends well beyond the edge of the visible galaxy, and it has roughly the same density everywhere, so over huge volumes that mass adds up, and up, and up, and turns out to be, in aggregate, greater than the mass of all of the ordinary matter in our galaxy, by a factor of about 5:1.