Dark matter is believed to be very weakly self interacting. If it had any type of self interaction it is assumed that it would aggregate into large structures (

Dark matter is believed to be very weakly self interacting. If it had any type of self interaction it is assumed that it would aggregate into large structures (like stars) because it would lose momentum due to collisions or "drag" interacting with other dark matter. We would expect to observe structures like dark matter stars/galaxies/planets, but no such structures are observed. Even if those large structures did not interact electromagnetically, the gravitational effects of those objects would be felt on "ordinary matter" like stars and galaxies that we could observe.