  Models for the formation and growth of structure in a cold dark matter
dominated universe predict that galaxy halos should contain significant
substructure. Studies of the Milky Way, however, have yet to identify the
expected few hundred sub-halos with masses greater than about 10^6 Msun. Here
we propose a test for the presence of sub-halos in the halos of galaxies. We
show that the structure of the tidal tails of ancient globular clusters is very
sensitive to heating by repeated close encounters with the massive dark
sub-halos. We discuss the detection of such an effect in the context of the
next generation of astrometric missions, and conclude that it should be easily
detectable with the GAIA dataset. The finding of a single extended cold stellar
stream from a globular cluster would support alternative theories, such as
self-interacting dark matter, that give rise to smoother halos.
