Dark matter halos are often found around galaxies, extending far past them. The dark matter fails to stay within the galaxy because it's not slowed down by regular interactions.
Also, dark matter is able to pull regular matter to be closer to it. If a galaxy's dark matter is moving too far away from a galaxy, it might pull the galaxy with it. (Or it might separate entirely, as happened with the Bullet Cluster.)
The universe didn't form uniformly, so if a bunch of dark matter formed as a clump, it would remain like that.
Also dark matter is gravitationally affected by light matter, which does interact. My intuition tells me that the interaction of light matter that creates clumps of light matter will also create clumps of dark matter indirectly.
* note: 90% my dark matter knowledge comes from HN. I'm just spitballing here
What happens to a comet that enters the solar system if it doesn’t hit anything? It leaves the solar system with the same speed it entered it with. Dark matter cannot hit anything.
