  Dark matter is the dominant form of matter in the universe, but its nature is
unknown. It is plausibly an elementary particle, perhaps the lightest
supersymmetric partner of known particle species. In this case, annihilation of
dark matter in the halo of the Milky Way should produce gamma-rays at a level
which may soon be observable. Previous work has argued that the annihilation
signal will be dominated by emission from very small clumps (perhaps smaller
even than the Earth) which would be most easily detected where they cluster
together in the dark matter halos of dwarf satellite galaxies. Here we show,
using the largest ever simulation of the formation of a galactic halo, that
such small-scale structure will, in fact, have a negligible impact on dark
matter detectability. Rather, the dominant and likely most easily detectable
signal will be produced by diffuse dark matter in the main halo of the Milky
Way. If the main halo is strongly detected, then small dark matter clumps
should also be visible, but may well contain no stars, thereby confirming a key
prediction of the Cold Dark Matter (CDM) model.
