Dark matter not really dark?

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Delta2
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If we could observe a piece of dark matter with naked eye would it appear dark or invisible? Dark matter doesnt emit photons at all but it neither absorbs photons (it doesnt interact at all with photons right?) so it would appear invisible?
 

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Orodruin
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I do not now how you imagine a "piece" of dark matter, it interacts very weakly also with itself and does not have macroscopic phases in the same way normal matter does. But yes, dark matter would be invisible to you, the way we know it is there is by looking at its gravitational effects, including gravitational lensing and rotation curves of galaxies.
 
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Delta2
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well to tell you the truth i thought black matter forms some sort of gigantic solid bodies, and when we look at them we see only the empty black space of the universe thats where the name dark comes from. But since you say it interacts very weakly with itself i guess dark matter is in some sort of gaseous phase and the only thing that does is creating gravitational field, absoletuly nothing else right?
 
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phinds
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well to tell you the truth i thought black matter forms some sort of gigantic solid bodies, and when we look at them we see only the empty black space of the universe thats where the name dark comes from. But since you say it interacts very weakly with itself i guess dark matter is in some sort of gaseous phase and the only thing that does is creating gravitational field, absoletuly nothing else right?
Hard to imagine where you got the idea that dark matter forms large solid bodies. I've never seen any such statement. Do you remember where you saw it?

"Dark" is the term used because it is literally dark ... it does not reflect or emit photons. As orodruin said, it interacts only weakly (if at all) not only with itself but also with normal matter, so there is no way for it to clump (that's what he was referring to by saying it has no macroscopic phases) so it doesn't form even small bodies, much less large ones. Still, it is immensely important to the formation of the universe as we know it.
 
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Delta2
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Hard to imagine where you got the idea that dark matter forms large solid bodies. I've never seen any such statement. Do you remember where you saw it?

"Dark" is the term used because it is literally dark ... it does not reflect or emit photons. As orodruin said, it interacts only weakly (if at all) not only with itself but also with normal matter, so there is no way for it to clump (that's what he was referring to by saying it has no macroscopic phases) so it doesn't form even small bodies, much less large ones. Still, it is immensely important to the formation of the universe as we know it.
I didnt saw that statement anywhere i guess i automatically thought it by myself.

But in order for the dark matter to be dark it has to absorb photons right? If the photons pass through it like it doesnt exist (except from the interaction of photons with dark matter's gravitational field) then dark matter would appear invisible not dark
 
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phinds
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I didnt saw that statement anywhere i guess i automatically thought it by myself.

But in order for the dark matter to be dark it has to absorb photons right? If the photons pass through it like it doesnt exist (except from the interaction of photons with dark matter's gravitational field) then dark matter would appear invisible not dark
That's certainly an interesting way of looking at it, and the "invisible" part sounds right to me. We still call dark matter dark because no photons get from it to us (either by emission or by reflection).
 
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phinds
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That's certainly an interesting way of looking at it, and the "invisible" part sounds right to me. We still call dark matter dark because no photons get from it to us (either by emission or by reflection).
EDIT: another thing to keep in mind is that intergalactic dust made of normal matter may have what we humans would consider dust-sized (and even larger) clumps but dark matter will not because it does not clump. If DM is in fact WIMPS, these are still very tiny and would be a lot less likely to impede photons than would normal matter dust. On the other hand, there must be a LOT of them to make up the mass of DM.
 
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Are dark matter bodies ruled out then? Only DM particles.
 
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ChrisVer
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Are dark matter bodies ruled out then? Only DM particles.
Do you mean objects like MACHOs? they contribute to the DM, but they are not enough...so, partially yes
 
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ChrisVer
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But in order for the dark matter to be dark it has to absorb photons right? If the photons pass through it like it doesnt exist (except from the interaction of photons with dark matter's gravitational field) then dark matter would appear invisible not dark
Dark Matter is Dark because we cannot see that. By "see" I don't only mean photons (is not a source). It's not "black". You can have a wall of dark matter let's say, but you cannot see that, and you can walk right through it as if didn't exist. The only thing that shows that DM exists is its gravity.
So no, it doesn't have to absorb photons. It only has to contribute to the gravity.
Neutrinos pass through the earth without a big problem (earth is transparent to neutrinos), does that mean to you that Earth doesn't exist? Or that neutrinos don't exist? In the same way, a very weakly interacting matter can exist, and in big scales it can solve some problems due to its gravity.
 
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  • #11
Chalnoth
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If we could observe a piece of dark matter with naked eye would it appear dark or invisible? Dark matter doesnt emit photons at all but it neither absorbs photons (it doesnt interact at all with photons right?) so it would appear invisible?
Dark matter doesn't interact with light hardly at all. The CMB evidence makes this pretty clear: before the CMB was emitted, the temperature was high enough that any normal matter would have been in the form of a plasma. But there is very clear evidence of lots of mass that couldn't interact with the plasma.
 
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Dark Matter does not emit nor absorb any light, it does not react electromagnetically, that is what makes it "the mystery".
 
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ChrisVer
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Dark Matter does not emit nor absorb any light, it does not react electromagnetically, that is what makes it "the mystery".
Or if it does interact Electromagnetically, it does so remaining unobserved (weakly interacting)...eg the axions
 
  • #14
Chronos
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If dark matter interacts only via gravity, we may never know what it is. All searches for dark matter are forced to assume it has other interactions, even if only with itself.
 
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Fascinating stuff for certain. Out of curiosity, does there exist any models in which dark matter could under special circumstances be able to form any sort of a "crystalline" structure, even maybe on scales as small as a handful of particles? Of course any such model would require that it does in fact interact weakly. But is there anything that outright precludes such interactions in DM?

For that matter, being very ignorant of terminology does reference to "weakly" interacting actually involve the force understood to be the weak force?
 
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Chronos
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Weakly interacting is like a neutrino being captured by a fly - highly unusual.
 
  • #17
ChrisVer
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For that matter, being very ignorant of terminology does reference to "weakly" interacting actually involve the force understood to be the weak force?
No it doesn't have to be the weak force. It can be any other type of interaction, it only has to be very weak.

Out of curiosity, does there exist any models in which dark matter could under special circumstances be able to form any sort of a "crystalline" structure, even maybe on scales as small as a handful of particles?
crystalline structure?
There are models in which cold dark matter can form gravitationally bound objects (eg. axion miniclusters due to inhomogenities)
 

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