  We study the formation of voids in a modified gravity model in which gravity
is generically stronger or weaker on large scales. We show that void abundances
provide complementary information to halo abundances: if normalized to the CMB,
models with weaker large-scale gravity have smaller large scale power, fewer
massive halos and fewer large voids, although the scalings are not completely
degenerate with $\sigma_8$. Our results suggest that, in addition to their
abundances, halo and void density profiles may also provide interesting
constraints on such models: stronger large scale gravity produces more
concentrated halos, and thinner void walls. This potentially affects the
scaling relations commonly assumed to translate cluster observables to halo
masses, potentially making these too, useful probes of gravity.
