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