Our current most favorable model of cosmology, the Lambda-CDM model has been shown to strongly match observations on cosmic (CMB), cluster, and group scales. On smaller scales, this agreement has been less clear. In particular, Lambda-CDM robustly predicts that all massive dark matter halos contain tens of thousands of smaller dark matter subhalos. Observations of satellite galaxies around the milky way have observed a factor of 10-100 deficit of subhalos than are predicted by numerical simulations of Milky Way-like dark matter halos. This discrepancy has been termed the missing satellites problem, and it is one of several remaining problems with Lambda-CDM on sub-galactic scales.
The missing satellites problem can be settled either by modifying the microphysics of dark matter in such a way that low mass subhalos do not form, or by invoking baryonic feedback that prevents stars from forming in subhalos, effectively making them 'dark subhalos.' These scenarios predict radically different shapes of the subhalo mass function, making a measurement of the mass function an attractive means of testing for new dark matter physics. Gravitational Lensing is the only current means of detecting dark matter subhalos and measuring the subhalo mass function outside of the Milky Way and is one of the avenues that offers the greatest promise of resolving this debate. This field has advanced rapidly in recent years with the advent of the Atacama Large Millimeter/sub-millimeter array (ALMA), an array of telescopes that can take images with unprecedented angular resolution and sensitivity. In this talk, I will describe the technique of using ALMA as the worlds most powerful dark matter subhalo detector. I will also showcase some of the recent results that ALMA lens modeling has produced and prophesize about how future ALMA endeavors will advance our understanding of dark matter.