Dark Shards: velocity substructure from Gaia and direct searches for
  dark matter
  Data from the Gaia satellite show that the solar neighbourhood of the Milky
Way's stellar halo is imprinted with substructure from several accretion
events. Evidence of these events is found in "the Shards", stars clustering
with high significance in both action space and metallicity. Stars in the
Shards share a common origin, likely as ancient satellite galaxies of the Milky
Way, so will be embedded in dark matter (DM) counterparts. These "Dark Shards"
contain two substantial streams (S1 and S2), as well as several retrograde,
prograde and lower energy substructures. The retrograde stream S1 has a very
high Earth-frame speed of $\sim 550$ km s$^{-1}$ while S2 moves on a prograde,
but highly polar orbit and enhances the peak of the speed distribution at
around $300$ km s$^{-1}$. The presence of the Dark Shards locally leads to
modifications of many to the fundamental properties of experimental DM signals.
The~S2 stream in particular gives rise to an array of effects in searches for
axions and in the time dependence of nuclear recoils: shifting the peak day,
inducing nonsinusoidal distortions, and increasing the importance of the
gravitational focusing of DM by the Sun. Dark Shards additionally bring new
features for directional signals, while also enhancing the DM flux towards
Cygnus.
