GOLDILOCKS would approve. It seems that dark matter 鈥� that stuff in the universe that we know is there but cannot see 鈥� is not too cold or too hot, but just right.
Astronomers at the University of Cambridge made detailed observations of 12 dwarf galaxies near the Milky Way to calculate how much dark matter must be holding them together. 鈥淣o matter what size, how bright, or how many stars they contained, all the galaxies seemed to be sitting in roughly the same amount of dark matter,鈥� says team member Mark Wilkinson. They found that dark matter clumps together in 鈥渂uilding blocks鈥�, the smallest of which is 1000 light years across with 30 million times the mass of the sun.
The size of the blocks narrows down the temperature of dark matter. If it were very hot 鈥� say, neutrinos travelling at close to the speed of light 鈥� gravity could not hold it together in blocks this small, according to team member Gerry Gilmore. On the other hand, cold, slow-moving dark matter would form much smaller clumps. That leaves us with tepid dark matter, and means that a special flavour of weakly interacting massive particles (WIMPs) 鈥� which hardly interact with ordinary matter but interact strongly with each other 鈥� are the most likely stuff of dark matter.
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