Does the halo mass function continue below $1e8\,\text{solar masses}$?
In plain words
Cold dark matter predicts huge numbers of small dark clumps with no stars in them. They can be detected only by how they bend light from distant objects or disturb thin streams of stars.
Precise statement
Measure the subhalo and line-of-sight halo mass function $dn/dM$ over $M = 1e6\text{ to }1e9\ M_{\mathrm{sun}}$. CDM predicts dn/dM ~ M^-1.9 with no cutoff above $\sim 1e-6\ M_{\mathrm{sun}}$; warm, fuzzy or interacting dark matter predict suppression below a half-mode mass $M_{\mathrm{hm}}$. An answer is a measured amplitude and slope or a bound on $M_{\mathrm{hm}}$ from strong-lens flux ratios, gravitational imaging and stellar-stream gaps, with the density profiles of detected clumps.
What would settle it
Gravitational imaging and flux-ratio statistics of hundreds of strong lenses from Euclid and Rubin with high-resolution follow-up, combined with Milky Way stream perturbation counts.
Status in the literature
Unverified note
A $1e6\,M_{\mathrm{sun}}$ dark object was detected by gravitational imaging in JVAS B1938+666 (Powell et al. 2025, arXiv:2510.07382); its high central density is debated as CDM versus core-collapsed self-interacting dark matter.