  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 rely 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.
