Rotating turbulence has been the subject of intense research for many years, but even the framework which should be used for its description is still unclear. Recently it was suggested that the formalism of wave turbulence should provide a reliable description of the flow field. This formalism describes the system as an ensemble of interacting waves. In a rotating system, this ensemble should be composed of Coriolis driven inertial waves that are solutions to the linearized rotating Navier-Stokes equation. Our measurements show that the kinetic energy is concentrated along the entire dispersion relation curve of inertial waves. This is the first experimental evidence for the existence of inertial wave turbulence in steady rotating turbulence.