Is Dark Matter Just Unseen Molecules?

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Discussion Overview

The discussion revolves around the nature of dark matter, specifically questioning whether it could simply be unseen molecules. Participants explore the implications of the Bullet Cluster observations and the characteristics of dark matter, including its interaction properties and theoretical candidates.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that the Bullet Cluster collision provides evidence for dark matter, questioning if it could be unseen molecules.
  • Others argue that dark matter is "conclusionless," meaning it does not interact with itself or normal matter, which would prevent any shock waves that would occur if normal molecules were involved.
  • A participant raises the question of whether neutrinos interact with each other, noting their low mass and implications for their presence in clusters.
  • There is a discussion about the annihilation cross section of dark matter, with some participants stating it is very small, leading to a lack of observed self-interaction effects.
  • One participant claims that if dark matter does not interact with itself or normal matter, it cannot be falsifiable, thus questioning its existence.
  • Another participant counters that neutrinos exist despite their minimal interaction with matter, highlighting the uncertainty surrounding dark matter's nature.
  • Some participants propose that dark matter should be cold and weakly interacting, which aligns with many exotic dark matter candidates, while noting it does not behave like gas.
  • There is a clarification regarding the term "cold," with one participant explaining it refers to low-velocity particles rather than literal temperature.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of dark matter, its interaction properties, and implications of the Bullet Cluster observations. The discussion remains unresolved, with no consensus on the underlying nature of dark matter.

Contextual Notes

Limitations include the lack of consensus on the properties of dark matter, the dependence on theoretical models, and the unresolved nature of its interaction characteristics.

touqra
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The observation on the bullet cluster collision was stated as direct evidence for dark matter.
http://apod.nasa.gov/apod/ap060824.html

But could there be a possibility that it's just some molecules that are too dark to be seen ?
 
Astronomy news on Phys.org
A key point about dark matter that is relavant to the Bullet Cluster result is that dark matter is conlussionless. This means that whatever dark matter is made from, it does not interact with anything, not even itself. This means two 'clouds' of dark matter can pass through each other within hinderance. Any 'normal' molecules would hit each other in this process, causing large shock waves.

In the Bullet Cluster result what you see is that the 'normal' matter in the clusters, the gas, forms these shock fronts and give of the X-rays observed, while the dark matter does not. This leads to a spatial separation of the dark and normal matter and a big headache for modified gravity theories!

There is no way known for any molecules of normal matter to act in the way we've observed dark matter to in this cluster system, given the required mass density the gravitational lensing result implies.
 
Do neutrinos interact with each other?
 
Riogho said:
Do neutrinos interact with each other?

Since they have no mass (or a REALLY tiny mass) they wouldn't stay in the cluster.
 
Wallace said:
A key point about dark matter that is relavant to the Bullet Cluster result is that dark matter is conlussionless. This means that whatever dark matter is made from, it does not interact with anything, not even itself. This means two 'clouds' of dark matter can pass through each other within hinderance. Any 'normal' molecules would hit each other in this process, causing large shock waves.

The cross section of dark matter annihilation is not zero, and with that powerful collision, there should be interaction among them ?
 
touqra said:
The cross section of dark matter annihilation is not zero, and with that powerful collision, there should be interaction among them ?

No one has any idea what the annihilation cross section of dark matter is, apart from a very small upper limit (i.e. whatever it is it is tiny). The reason we know it must be small is precisely because we don't see any self interaction effects.
 
Wallace said:
No one has any idea what the annihilation cross section of dark matter is, apart from a very small upper limit (i.e. whatever it is it is tiny). The reason we know it must be small is precisely because we don't see any self interaction effects.

If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?
 
touqra said:
If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?

This isn't true. Neutrino's exist, but they don't interact with matter (at least not much!), nor with each other.

Also, problems arise because we don't actually know what dark matter is. It's only something theorized because atoms apparently only make up ~4% of the universe.
 
touqra said:
If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?
It does interact via gravity, which is what opened the whole can of worms in the first place.
 
  • #10
touqra said:
If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?

I think the message of bullet cluster observation is more that dark matter should be rather cold (in order to condense in galaxies) and weakly interacting (like most of exotic dark matter candidates).
So this leaves lots of candidates.
But, it is not behaving as gas which is already a very important news.
 
  • #11
Barmecides said:
I think the message of bullet cluster observation is more that dark matter should be rather cold (in order to condense in galaxies) and weakly interacting (like most of exotic dark matter candidates).
So this leaves lots of candidates.
But, it is not behaving as gas which is already a very important news.

It's not that dark matter should be cold - but more interestingly just that it doesn't glow/light up if you warm it.
 
  • #12
mikehibbert said:
It's not that dark matter should be cold - but more interestingly just that it doesn't glow/light up if you warm it.

Hello mike,

by cold, I was meaning it is not particle of high velocity like neutrinos which are often considered as hot dark matter candidates and which dilute too quickly.
What did you mean ?
 
  • #13
Ah ok, I thought you meant in a literal temperature sense.

Silly me! :)
 

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