We use an extended Tanh-function expansion method to solve the nonlinear equation for ultrashort pulse propagation in matamaterial，and obtain dark solitary solutions in various cases. Further，we study the influence of the controllable self-steepening effect and the second-order nonlinear dispersion on the formation and propagation of dark solitons in metamaterials. It is found that the negative self-steepening effect in metamaterial makes the soliton center move to the leading side，opposite to the moving direction of soliton in ordinary materials in which the self-steepening effect is always positive. Most importantly，due to the role of the second-order nonlinear dispersion，dark solitons can be formed in the medium without linear dispersion or with an anomalous linear dispersion.