  In this Letter we explore the effects of the scattering of photons incident
on a dark matter halo through their interaction with either electrons or
photons produced by dark matter annihilation. Particularly, we examine the
effects of this scattering upon the observed spectrum of a distant AGN or of
the Cosmic Microwave Background. Our results indicate that ultra-compact mini
halos and other dense dark matter clumps can induce an observable
Comptonisation of AGN spectra as well as a Sunyaev-Zel'dovich effect (SZE) with
an optical depth similar to that attained by thermal electrons in the Coma
cluster. The rate of encounters between a distant AGN and these dense
mini-halos is also estimated using micro-lensing limits existing on the
population of dark compact bodies.
