Dark matter nude - 'Groupie' galaxies orbiting Milky Way tell us about dark matter, how galaxy formed

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The further away from the centre of the galaxy the object lies, the more susceptible it is to surrounding dark matter, and the faster it moves.

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Note how planetary nebulae can be detected well beyond the apparent edge of the galaxy, and that the dots tend to get smaller far from the galaxy, indicative of slow speeds and hence a lack of dark matter.