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Evidence for dark matter

  1. Feb 14, 2008 #1
    The observation on the bullet cluster collision was stated as direct evidence for dark matter.

    But could there be a possibility that it's just some molecules that are too dark to be seen ?
  2. jcsd
  3. Feb 15, 2008 #2


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    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 seperation 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.
  4. Feb 15, 2008 #3
    Do neutrinos interact with each other?
  5. Feb 15, 2008 #4
    Since they have no mass (or a REALLY tiny mass) they wouldn't stay in the cluster.
  6. Feb 15, 2008 #5
    The cross section of dark matter annihilation is not zero, and with that powerful collision, there should be interaction among them ?
  7. Feb 16, 2008 #6


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    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.
  8. Feb 16, 2008 #7
    If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
  9. Feb 16, 2008 #8
    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.
  10. Feb 16, 2008 #9


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    It does interact via gravity, which is what opened the whole can of worms in the first place.
  11. Feb 18, 2008 #10
    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.
  12. Feb 18, 2008 #11
    It's not that dark matter should be cold - but more interestingly just that it doesn't glow/light up if you warm it.
  13. Feb 19, 2008 #12
    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 ?
  14. Feb 19, 2008 #13
    Ah ok, I thought you meant in a literal temperature sense.

    Silly me! :)
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