> Why doesn't dark mater clump together to form dark galaxies? Because of the weak nature of its interactions - it barely interacts with itself, never mind bin

> Why doesn't dark mater clump together to form dark galaxies? Because of the weak nature of its interactions - it barely interacts with itself, never mind binding into local blobs. You can imagine starting with a big sphere of dark matter; under gravity it will collapse inwards towards its centre, but although it falls to a small region it just passes through itself and goes right out the other side again...oscillating right back to something close to the initial sphere. This is in contrast to 'normal' matter, which strongly interacts with itself and so will not pass through itself - instead it can bind and (through a range of physical effects) become planets, stars, or other things. Or at least heat up and redistribute energy in more interesting ways. > Do we see dark matter's influence in the motions of the stars in the milky way? Yes, you can see e.g. https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dark_matter_halo https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dar... for some information.