  I discuss the prospects of detecting the smallest dark matter bound
structures present in the Milky Way by searching for the proper motion of
$\gamma$-ray sources in the upcoming GLAST all sky map. I show that for dark
matter particle candidates that couple to photons the detection of at least one
$\gamma$-ray microhalo source with proper motion places a constraint on the
couplings and mass of the dark matter particle. For SUSY dark matter, proper
motion detection implies that the mass of the particle is less than 500 GeV and
the kinetic decoupling temperature is in the range of [4-100] MeV.
