From other threads here I’ve gained a rough understanding of why Dark Matter forms into a halo around galaxies. It makes sense, since DM doesn’t interact and doesn’t clump together, that when a DM particle falls from the halo towards the gravitational centre of the galaxy it gradually accelerates all the way up to very high speeds near the galactic centre. Once it has passed through this central region it then decelerates on the way out again and returns back to the halo with very little momentum, where eventually it will start to fall towards the galaxy centre again. And in order for the halo to exist to the extent it does, each DM particle must spend more time in the halo than travelling across the galaxy. I’m not sure I’ve got the last bit right though since 1) our galaxy is 100,000+ light-years across, 2) the halo is said to be far outside this limit and 3) the DM particle will only be travelling at a sizeable fraction of light speed for a small percentage of its journey. With these factors in mind I’d expect a DM particle to take several 100,000s (if not millions) of years between leaving and returning to the halo. So is it correct that it spends more time in the halo than out, and what are the current best estimates for the time scales of these oscillations? Also, on a more local scale, DM is said to be all around us, with roughly uniform distribution but at low density. I’m just wondering if this local DM consists entirely of that which is travelling across our galaxy or whether some of the DM has been permanently trapped by our sun’s local gravity well? I ask because I could imagine that by the time a DM particle gets to our solar system it has built up enough momentum to escape the ‘pull’ of the sun. If that is the case, are there any stars that DM particle encounter before they have gained ‘escape momentum’ (i.e. large stars, close to the galaxy rim) around which we might expect to find miniature DM halos?