Dark matter particles may decay, emitting photons. Drawing on the EAGLE family of hydrodynamical simulations of galaxy formation - including the APOSTLE and C-EAGLE simulations - we assess the systematic uncertainties and scatter on the decay flux from different galaxy classes, from Milky Way satellites to galaxy clusters, and compare our results to studies of the 3.55 keV line. We demonstrate that previous detections and non-detections of this line are consistent with a dark matter interpretation. For example, in our simulations the width of the dark matter decay line for Perseus-analogue galaxy clusters lies in the range of 1300-1700 km s−1 , and exceptionally up to 3000 km s−1 . Therefore, the non-detection of the 3.55 keV line in the centre of the Perseus cluster by the Hitomi collaboration is consistent with detections by other instruments. We also consider trends with stellar and halo mass and evaluate the scatter in the expected fluxes arising from the anisotropic halo mass distribution and from object-to-object variations. We provide specific predictions for observations with XMM-Newton and with the planned X-ray telescopes XRISM and ATHENA. If future detections of unexplained X-ray lines match our predictions, including line widths, we will have strong evidence that we have discovered the dark matter.