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The stars on the outer edges of galaxies go around faster than they should be. How is dark matter meant to account for this? if you just add more mass wont that just give it a greater gravitational force? So we would still expect the outer stars of galaxies to be going slower than the ones near the center. Whats the flaw in my reasoning?

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marked as duplicate by John Rennie, Kyle Kanos, Brandon Enright, ACuriousMind, Qmechanic Sep 5 '14 at 17:40

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  • $\begingroup$ It's the distribution of the mass that counts. If you bung a load of extra mass at the centre of the galaxy then it will indeed just increase the overall gravitational force. However the dark matter is spread throught the disk and extends outwards beyond it. This is explained in the question I've suggested as a duplicate. $\endgroup$ – John Rennie Sep 5 '14 at 16:36
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    $\begingroup$ possible duplicate of How does dark matter halo outside a galaxy help to explain galaxy rotation curve? $\endgroup$ – John Rennie Sep 5 '14 at 16:37
  • $\begingroup$ So there is more dark matter the further away from the center? If this is the case why? why isn't it just randomly distributed? $\endgroup$ – Ray Kay Sep 6 '14 at 3:06
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Yes, more mass would mean a greater gravitational pull. A greater gravitational pull, in turn, means that an object would have to be going faster at a particular distance if it is moving in a circle. Similar to an object tied to a string and going in a circle: the faster it's moving, the larger the required force (in this case tension).

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