http://news.bbc.co.uk/1/hi/sci/tech/4679220.stm Gerry Gilmore's Cambridge team did a survey of clouds of dark matter, using several telescopes including VLT in Chile. they found no blobs of DM smaller than about 30 million solar masses, or less than 1000 lightyear diameter. They inferred from this a characteristic TEMPERATURE of dark matter. the particles must be moving on average 9 kilometer per second, so that clouds that are not big enough DISPERSE by random motion. The clouds that are not big enough don't have enough gravity to hold themselves together, given the particles' speed of random motion.