  In many cosmologies dark matter clusters on sub-kiloparsec scales and forms
compact subhalos, in which the majority of Galactic dark matter could reside.
Null results in direct detection experiments since their advent four decades
ago could then be the result of extremely rare encounters between the Earth and
these subhalos. We investigate alternative and promising means to identify
subhalo dark matter interacting with Standard Model particles: (1) subhalo
collisions with old neutron stars can transfer kinetic energy and brighten the
latter to luminosities within the reach of imminent infrared, optical, and
ultraviolet telescopes; we identify new detection strategies involving
single-star measurements and Galactic disk surveys, and obtain the first bounds
on self-interacting dark matter in subhalos from the coldest known pulsar, PSR
J2144-3933, (2) subhalo dark matter scattering with cosmic rays results in
detectable effects, (3) historic Earth-subhalo encounters can leave dark matter
tracks in paleolithic minerals deep underground. These searches could discover
dark matter subhalos weighing between gigaton and solar masses, with
corresponding dark matter cross sections and masses spanning tens of orders of
magnitude.
