  The end state of Hawking evaporation of a black hole is uncertain. Some
candidate quantum gravity theories, such as loop quantum gravity and asymptotic
safe gravity, hint towards Planck sized remnants. If so, the Universe might be
filled with remnants of tiny primordial black holes, which formed with mass
$M<10^9\,{\rm g}$. A unique scenario is the case of $M\sim 5\times10^5\,{\rm
g}$, where tiny primordial black holes reheat the universe by Hawking
evaporation and their remnants dominate the dark matter. Here, we point out
that this scenario leads to a cosmological gravitational wave signal at
frequencies $\sim 100{\rm Hz}$. Finding such a particular gravitational wave
signature with, e.g. the Einstein Telescope, would suggest black hole remnants
as dark matter.
