  New data from the $\textit{Gaia}$ satellite, when combined with accurate
photometry from the Pan-STARRS survey, allow us to accurately estimate the
properties of the GD-1 stream. Here, we analyze the stellar density
perturbations in the GD-1 stream and show that they cannot be due to known
baryonic structures like giant molecular clouds, globular clusters, or the
Milky Way's bar or spiral arms. A joint analysis of the GD-1 and Pal 5 streams
instead requires a population of dark substructures with masses $\approx
10^{7}$ to $10^9 \ M_{\rm{\odot}}$. We infer a total abundance of dark subhalos
normalised to standard cold dark matter $n_{\rm sub}/n_{\rm sub, CDM} = 0.4
^{+0.3}_{-0.2}$ ($68 \%$), which corresponds to a mass fraction contained in
the subhalos $f_{\rm{sub}} = 0.14 ^{+0.11}_{-0.07} \%$, compatible with the
predictions of hydrodynamical simulation of cold dark matter with baryons.
