A fundamental prediction of the cold dark matter cosmology is the existence
of a large number of dark subhalos around galaxies, most of which should be
entirely devoid of stars. Confirming the existence of dark substructures stands
among the most important empirical challenges in modern cosmology: if they are
found and quantified with the mass spectrum expected, then this would close the
door on a vast array of competing theories. But in order for observational
programs of this kind to reach fruition, we need robust predictions. Here we
explore substructure predictions for lensing using galaxy lens-like hosts at
z=0.2 from the Illustris simulations both in full hydrodynamics and dark matter
only. We quantify substructures more massive than ~ 10^9 M_sun, comparable to
current lensing detections derived from HST, Keck, and ALMA. The addition of
full hydrodynamics reduces the overall subhalo mass function by about a factor
of two. Even for the dark matter only runs, most (~ 85%) lines of sight through
projected cylinders of size close to an Einstein radius contain no
substructures larger than 10^9 M_sun. The fraction of empty sight lines rises
to ~ 95% in full physics simulations. This suggests we will likely need
hundreds of strong lensing systems suitable for substructure studies, as well
as predictions that include the effects of baryon physics on substructure, to
properly constrain cosmological models. Fortunately, the field is poised to
fulfill these requirements.Comment: 11 pages, 9 figure
