  Very weakly interacting slim particles (WISPs), such as axion-like particles
(ALPs) or hidden photons (HPs), may be non-thermally produced via the
misalignment mechanism in the early universe and survive as a cold dark matter
population until today. We find that, both for ALPs and HPs whose dominant
interactions with the standard model arise from couplings to photons, a huge
region in the parameter spaces spanned by photon coupling and ALP or HP mass
can give rise to the observed cold dark matter. Remarkably, a large region of
this parameter space coincides with that predicted in well motivated models of
fundamental physics. A wide range of experimental searches -- exploiting
haloscopes (direct dark matter searches exploiting microwave cavities),
helioscopes (searches for solar ALPs or HPs), or light-shining-through-a-wall
techniques -- can probe large parts of this parameter space in the foreseeable
future.
