As far as I know galaxies and clusters form along so-called "galactic filaments" which are influenced by a dark matter web that organises them in to this position in the universe. So it looks like dark matter attracts baryonic matter by a kind of gravity. But is it also true the other way around, and that 'our' mass has gravitational forces on dark matter? And if not why does it only go one way?

In a study scientists have observed the four colliding galaxies of cluster Abell 3827 and discovered that one of the envelopes of dark matter in the galaxy behind it encloses. The backlog in the system stands at 5,000 light years (50,000 million million kilometers - the NASA space probe Voyager would do 90 million years to cover that distance. Expected to create some lag between a galaxy and its associated dark matter when dark matter affects itself, however small, by other forces than the gravity.

See also https://www.youtube.com/watch?v=qtfTUPhXutQ and http://spacetelescope.org/news/heic1506/


Dark matter attracts baryons not only by a "kind of gravity", but simply by gravity. Everything that has mass (even if it's only relativistic mass like light) attracts everything else by gravity, and thus baryons also attract dark matter.

The reason dark matter "organizes" galaxies is just that there is much more of it, roughly 7 times more, to be specific. So, on large scales, the dynamics of galaxies are dominated by dark matter. But because baryons can dissipate energy, it may collapse in clumps — such as molecular clouds — which are hence dominated by baryons.

  • $\begingroup$ well maybe! Given we haven't actually "seen" any dark matter it's maybe best to say the evidence suggests that there is about 7x more of it than "not dark" matter. $\endgroup$ Mar 6 '16 at 21:07
  • $\begingroup$ The question assumed that dark matter explains structure formation. No need to justify it in the answer. $\endgroup$
    – ProfRob
    Mar 6 '16 at 22:28
  • 3
    $\begingroup$ @adrianmcmenamin We "see" dark matter by its gravitational effect on matter. If you insist on literal seeing, we have not "seen" a lot of things we think of as having some mode existence. A recent example would be the detection of gravity waves, we have not seen them but we have "seen" them. For that matter (pun intended) does seeing a structure on an image on a screen count as seeing the entity of which we think it an image if there is no possibility of seeing this with the unaided eye? $\endgroup$ Mar 7 '16 at 8:17
  • $\begingroup$ I don't which is why I put "seen" in inverted commas. But we haven't seen anything in the LHC or anywhere else either. We've just seen a force and to explain that we posit this "dark matter" and then start guessing what it is but no other experiments (afaik) have confirmed any of that, that's all I am suggesting. $\endgroup$ Mar 7 '16 at 16:29

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