Sterile neutrinos with masses in the kilo-electronvolt range are viable candidates for the warm dark matter. We analyse existing data for the extragalactic diffuse X-ray background for signatures of sterile neutrino decay. The absence of a detectable signal within current uncertainties of background measurements puts model-independent constraints on allowed values of the sterile neutrino mass and mixing angle, which we present in this work. © 2006 RAS.