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Aseyhe t1_iu8tdpt wrote

Reply to comment by elite4koga in Is dark matter orbiting galaxies with the same speed as normal matter? by taracus

Sure, it holds for almost every dark matter model that we consider, including

  • cold dark matter
  • warm dark matter
  • fuzzy dark matter (which is so light that quantum effects are important)
  • primordial black holes or other massive objects
  • self-interacting dark matter (with elastic collisions)

There are, however, "dissipative" dark matter models that are capable of losing energy through inelastic collisions. In those models, there would be a tendency to form disks over very long time scales. Since we know that dark matter halos are much more broadly extended than galaxies, the time scale for dark disk formation must be a lot longer than the time scale to form galactic disks, though.

Also, a dark matter particle that has a significant nongravitational interaction with ordinary matter could be dragged along with the disk. It's hard to reconcile this possibility with (1) absence of dark matter direct detection and (2) absence of dark matter creation in colliders, though.

> to my knowledge there isn't even strong evidence of where its located.

We have a fairly precise picture, e.g. https://academic.oup.com/mnras/article/494/3/4291/5821286

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elite4koga t1_iu94rbt wrote

This is great info, exactly what I was looking for. Thanks for the follow up!

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TastiSqueeze t1_iu9ia3a wrote

In some cases dark matter has been separated from the galaxy it was associated with and in a few cases is believed to rotate in the opposite direction as the host galaxy.

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Goodpie2 t1_iua7ga5 wrote

How do we even know that?

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ViscountTinew t1_iuagdu0 wrote

Gravitational lensing is one way - even if we can't see the dark matter, we can notice the lensing of light passing around it if there's enough of the stuff.

The obvious example is the Bullet Cluster. It's two galaxies that collided with and passed through each other. It's been a while since the collision so there's a fair amount of space in between the two galaxies.

What's interesting about the bullet cluster is that the lensing effects appear to be focused around a point in between the two galaxies, in what appears to be empty space, rather than the galaxies themselves, implying a large invisible mass of dark matter that has been left behind since the collision.

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PM_Me_Frosted_Tits t1_iuar47i wrote

How do we know that the mass is, in fact, dark matter?

What methods do they have at their disposal ruled out, say... A large field of interstellar medium that has coalesced since the event and wasn't dispersed by it for some reason unknown to the current model of galactic collisions?

From my understanding we haven't technically proven dark matter's existence yet. We see the effects of something and use the term "dark matter" to explain it, but do we know for sure that it isn't a secondary effect of something we can currently detect?

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ViscountTinew t1_iub3gg1 wrote

Because whatever the mass is, it isn't emitting any light or blocking any light. Everything has a temperature and normal matter should be detectable in some wavelength or another, especially in the amount needed to create the lensing effects. A cloud of dust and gas massive enough to cause the lensing would be easily visible in the radio/far-infrared spectrums.

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PM_Me_Frosted_Tits t1_iubbkah wrote

I figured there was some reaction I just wasn't aware of that they've used, I didn't even think of things like friction between molecules as being enough to detect against the background of space at that distance.

I appreciate it, this is what I was hoping someone would be able to explain.

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Natural_Pressure6329 t1_iubfq4w wrote

This brings a question to my mind. There is a “neutral” point between two masses, it’s the point at which the two gravitational forces sort of level out (I forgot the proper name), could this be the cause of the lensing being centered there?

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ryry1237 t1_iubg437 wrote

Presumably any earlier calculations would already be done with this in mind and the math doesn't add up with just ordinary matter.

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Putnam3145 t1_iuayg6d wrote

Interstellar medium undergoes more friction than our observations would allow.

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PM_Me_Frosted_Tits t1_iub1rf9 wrote

Meaning we'd see something in the Infrared range because of heat produced during friction?

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