Discovering the Dark Side of CDM Substructure

The existence of mass substructure is an inevitable consequence of the LCDM paradigm. Depite this clear theoretical prediction, a glaring discrepancy with the number of luminous satellites around the Milky Way {MW} has been identified, i.e. the "missing satellite problem". Notwithstanding great progress to address this problem, it remains unsolved. Moreover, the MW could be a biased environment, not representative of the typical universe. Surface brightness aberrations of gravitationally lensed images provide a complementary channel to detect substructure beyond the local Universe in mostly massive early-type galaxies. Simulations suggest a substructure mass fraction of f_sub99% CL. We request 30 orbits with HST WFC3-F390W to accomplish this goal.