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Dark matter was needed to give the stars in a spiral galaxy a cause of their speed. So they proposed there must be some extra dark matter in the Galaxy. The total extra mass of dark mater is quite high. As a consequence of that, the whole galaxy becomes much heavier. But in the group or a cluster, the Galaxy also has a speed relative to other galaxies. So my question is whether the mass needed to give the stars their speed is the same mass that is needed to explain its movement within its cluster or a group?

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    $\begingroup$ I'm not sure what you're asking. Certainly the dark matter mass affects the relative speed of galaxies in our local group. $\endgroup$
    – userLTK
    Jan 21 '16 at 0:29
  • $\begingroup$ I'm understanding the question as whether the dark matter mass within the galaxies explaining the motion of individual stars is consistent with the motion of the whole galaxies in the galaxy cluster. I don't know the answer for sure, but I think, there is needed additional dark matter outside the galaxies to account for the motion of the galaxies in the cluster. $\endgroup$
    – Gerald
    Jan 22 '16 at 0:59

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