Dark Matter (DM) is still hypothetical at this point. It is proposed to be something like ordinary matter in that its mass warps space-time, but has so little (or no) interaction with ordinary matter that we don't see any. The "clumping" question is interesting because we would expect dark matter to be drawn together by gravity. The Earth is not (currently) collapsing into a black hole because the atoms it is made of interact with each other to push against that. We don't know what would happen in the dark matter case. Just because DM has no or almost no interaction with ordinary matter does not mean it does not interact with itself through forces that we know nothing about. If DM does not push back against itself we would expect gravity to collect any local concentration of DM (not diffuse enough to be expanding apart by dark energy) and quickly collapse that into a black hole, if even only a tiny one. If DM does not "clump" the question why it doesn't would be a tough one.

So, all of this makes it very interesting to do experiments to look for weakly interacting particles (WIMPs), even though they are, by definition, very unlikely to show up at any given moment. Alternate hypothesis exist. There may be no DM, at all, and something else is causing the ripples in space-time we observe and attribute to DM. Gravity keeps showing different levels of complexity as we get more observations, as it did in our scientific history going from Galileo to Newton to Einstein. The wonderful and amazing part is that in just a few hundred years we have come from timing balls rolling down ramps to timing star systems rolling down gravitation arcs to black holes at the centers of galaxies, and even weighing entire clusters of galaxies. Wow.