Thin halo streams are the Galaxy's most delicate instruments. GD-1 and the Palomar 5 stream are ribbons of stars tens of parsecs wide with internal velocity dispersions of a few kilometers per second, tracing near-great-circle orbits through the halo (Odenkirchen et al. 2003; Grillmair and Dionatos 2006). Their fragility is their value: anything massive passing nearby leaves a permanent scar, making the streams sensitive detectors of the dark substructure ΛCDM requires.
The model's prediction is specific: thousands of dark subhalos, the small end of the CDM mass spectrum, should orbit the Milky Way and repeatedly buzz the streams, cumulatively producing gaps, kinks, fanning, and thickening at calculable rates (Yoon, Johnston and Hogg 2011). The observations cut both ways and neither comfortably: GD-1 and Pal 5 show some density variations and gaps, but disentangling subhalo scars from the known baryonic perturbers, the Galactic bar, spiral arms, giant molecular clouds, and the streams' own epicyclic overdensities, has proven treacherous, and analyses run from claims of subhalo-rate consistency to constraints that undercut the predicted abundance (Bovy et al. 2017). Decades into the program, the streams have yet to deliver an unambiguous dark-subhalo detection, the single most direct small-scale prediction of particle dark matter.
The standing is an unfinished decisive experiment: Gaia and LSST will map stream perturbations at far higher fidelity, and Rubin-era stream discoveries will multiply the detector count.