Dark matter is as mysterious as it is ubiquitous. Cosmological evidence raises more questions than it answers about its origin and nature. The only thing measured about dark matter is how much is out there. This measured abundance is a surprisingly powerful clue to dark matter's constituents and how they interact with us. Inspired by this clue, recent years have seen a growing interest in broadening the search for dark matter, in particular looking for ``light dark matter'' candidates --- those with masses between the electron and proton mass scales --- in addition to the heavier ``WIMP'' dark matter that dominated searches for the last 3 decades. I will discuss the theoretical underpinnings of light dark matter and the intriguing possibility that it could be our first window into a ``dark sector'' of new particles and interactions hiding under our noses. I will also explain the challenges and prospects for detecting light dark matter in the near future. I will focus in particular on the proposed Light Dark Matter eXperiment (LDMX) at SLAC — a search for dark matter production in the scattering of energetic electrons in a fixed target.