Some progress & challenges for the direct-detection of sub-GeV dark matter

In this contribution to the special issue on the Nobel Symposium on Dark Matter (NS 182), I review some recent theoretical and experimental progress, as well as some current challenges, in searching for dark matter particles below the proton mass with low-threshold direct-detection experiments. We now have several detection concepts for probing sub-GeV dark matter, and several technologies exist that can measure the small signals that such particles would produce in the detector. We have also made significant progress in calculating the dark matter interaction rates in crystals. In addition, we have discovered new signals for dark matter, and I will focus on the boosted dark matter component that is produced in the Sun. I will also discuss a recent (unsuccessful) attempt to probe, for the first time, the 'Migdal' effect in liquid xenon in a neutron scattering experiment. Moreover, I will highlight the discovery of several novel experimental backgrounds that mimic the expected dark matter signal; however, other backgrounds remain to be understood. Finally, I will discuss an exciting new detector concept, the dual-sided Charge-Coupled Device (CCD).