We study exciton-polariton nonlinear optical fluids in the high momentum waveguide regime for the first
time. We demonstrate the formation of dark solitons with the expected dependence of width on fluid
density for both main classes of soliton-forming fluid defects. The results are well described by numerical
modeling of the fluid propagation. We deduce a continuous wave nonlinearity more than ten times that on
picosecond time scales, arising due to interaction with the exciton reservoir
