  It is possible that a multi-component dark matter model is required if
primordial black holes only contribute to a fraction of the energy density in
dark matter. This is increasingly more likely with respect to the case of
$f_{\rm PBH} = 1$, since there is only one remaining window, on asteroid-mass
scales, where primordial black holes can make up all of the dark matter. A
mixed dark matter model can lead to interesting observables that come about due
to the interactions between primordial black holes and the second dark matter
component. This can provide unique signatures of the presence of primordial
black holes and increase the prospects of detection or improvement of
constraints in the mass ranges where $f_{\rm PBH} < 1$, whilst simultaneously
exploring the remaining open parameter space for other dark matter candidates.
