Something that is only affected by gravity won't tend to collect in one place because whatever gravity would attract it there also gives it momentum to get out.

Something that is only affected by gravity won't tend to collect in one place because whatever gravity would attract it there also gives it momentum to get out. You need collisions to get things to lose momentum to heat and clump together. Normal matter has collisions via interaction with the electromagnetic field, dark matter doesn't. Neutrinos also don't clump together, but are disqualified as dark matter candidates due to there not being enough of them; something about the relative abundance of different fundamental particles (e.g. electrons and neutrons) being important to get the universe we observer today. ETA: modified gravity doesn't give anything we can experiment on either; the difference of TeVeS[1] vs GR are not detectable at non-cosmological scales. [1] https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...