Dark matter only simulations of galaxy formation predict many more subhalosaround a Milky Way like galaxy than the number of observed satellites. Proposedsolutions require the satellites to inhabit dark matter halos with massesbetween one to ten billion solar masses at the time they fell into the MilkyWay. Here we use a modelling approach, independent of cosmological simulations,to obtain a preinfall mass of 360 (+380,-230) million solar masses for one ofthe Milky Way's satellites: Carina. This determination of a low halo mass forCarina can be accommodated within the standard model only if galaxy formationbecomes stochastic in halos below ten billion solar masses. Otherwise Carina,the eighth most luminous Milky Way dwarf, would be expected to inhabit asignificantly more massive halo. The implication of this is that a populationof "dark dwarfs" should orbit the Milky Way: halos devoid of stars and yet moremassive than many of their visible counterparts.
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