The effects of the derivative interactions in beyond-generalized Proca theory on the cosmic expansion history are investigated, and it is found that this may provide a theory of dark energy (with equation of state below -1) which agrees with redshift-space distortion data but which is distinguishable from the usual generalized Proca theory, or from the standard LCDM model. The constraints of ghost-free tensor, vector and scalar perturbations on a cosmic background are derived and are used to constraint the theory's parameter space.

Vector-tensor theories with at most second-order equations of motion are known as generalized Proca theories. Beyond-generalized Proca theory contains derivative interactions which, although contributing higher derivatives to the equations of motion, they do so in a degenerate way - no new degrees of freedom are excited (and so significantly, there is no Ostrogradski ghost). Just as beyond-Horndeski theories may provide novel cosmic expansion histories, it is natural to ask whether a dark vector sector might leave an observable imprint on the cosmic growth history, and whether the region of parameter space allowed by current observations match the region corresponding to a ghost-free, self-consistent theory.