We use the distribution of accreted stars in SDSS- Gaia DR2 to demonstrate that a non-trivial fraction of the dark matter halo within Galactocentric radii of 7.5–10 kpc and |z| > 2.5 kpc is in substructure , and thus may not be in equilibrium.Using a mixture likelihood analysis , we separate the contributions of an old , isotropic stellar halo and a younger anisotropic population .
The latter dominates and is uniform within the region studied .
It can be explained as the tidal debris of a disrupted massive satellite on a highly radial orbit , and is consistent with mounting evidence from recent studies .
Simulations that track the tidal debris from such mergers find that the dark matter traces the kinematics of its stellar counterpart .
If so , our results indicate that a component of the nearby dark matter halo that is sourced by luminous satellites is in kinematic substructure referred to as debris flow .
These results challenge the Standard Halo Model , which is discrepant with the distribution recovered from the stellar data , and have important ramifications for the interpretation of direct detection experiments .