The standard picture of cosmic structure formation rests on the cold dark matter (CDM) paradigm, in which structure grows hierarchically: small dark matter halos collapse first and later merge and accrete material to build larger systems. Galaxies form within these halos and co-evolve with their hosts. A direct consequence is the prediction of a vast population of low-mass halos in the present-day universe. Detecting them, however, is challenging. Beyond the Milky Way, strong gravitational lensing currently provides the only direct way to find such “little dark dots,” by revealing their subtle perturbations to lensed images. In this talk, I will present the techniques and state of the art in precision lens modeling for subhalo and line-of-sight halo detection. I will also discuss the key systematics and how we control them. Finally, I will outline prospects in the JWST, CSST, and Euclid era, where high-resolution imaging and large, homogeneous samples will transform these measurements into stringent tests of CDM and alternative dark-matter scenarios.
