Ordinary matter forms dense clumps like planets and stars because it's sticky. Atoms may be attracted to one another by gravity, but in order to get them to act

Ordinary matter forms dense clumps like planets and stars because it's sticky. Atoms may be attracted to one another by gravity, but in order to get them to actually stick together into dust grains, planets, stars et cetera, you need them to collide inelastically, and that requires the electromagnetic force. Two dark-matter particles in the void will be attracted to each other (slightly) by gravity, but when they meet they'll just sail straight on through one another without colliding. This prevents the formation of small-scale (ie sub-galactic scale) dark matter clumps.