  In the dark dimension scenario, which predicts an extra dimension of micron
scale, dark gravitons (KK modes) are a natural dark matter candidate. In this
paper, we study observable features of this model. In particular, their decay
to standard matter fields can distort the CMB and impact other astrophysical
signals. Using this we place bounds on the parameters of this model. In
particular we find that the natural range of parameters in this scenario is
consistent with these constraints and leads to the prediction that the mean
mass of the dark matter today is close to a few hundred keV and the effective
size of the extra dimension is around $1 - 30 \;\mu\mathrm{m}$.
