We consider the nonlocal Gross-Pitaevskii equation that models a Bose gas with general nonlocal interactions between particles in one spatial dimension, with constant density far away. We address the problem of the existence of traveling waves with nonvanishing conditions at infinity, i.e. dark solitons. Under general conditions on the interactions, we prove existence of dark solitons for almost every subsonic speed. Moreover, we show existence in the whole subsonic regime for a family of potentials. The proofs are based on a Mountain Pass argument combined with the so-called "monotonicity trick", as well as on a priori estimates for the Palais-Smale sequences. Finally, we establish properties of the solitons such us exponential decay at infinity and analyticity.