Evidence of a population of dark subhalos from Gaia and Pan-STARRS observations of the GD-1 stream

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 Ways 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.