  The general theory of dark solitons relies on repulsive interactions and
therefore predicts the impossibility to form dark-soliton bound states. One
important exception to this prediction is the observation of bound solitons in
non-local nonlinear media. Here, we report that exciton-polariton superfluids
can also sustain dark-soliton molecules although the interactions are fully
local. With a novel all optical technique, we create two dark solitons and bind
them to each other to form an unconventional dark-soliton molecule. We
demonstrate that the stability of this structure and the separation distance
between two dark-solitons is tightly connected to the driven-dissipative nature
of the polariton fluid.
