The Standard Model of particle physics leaves open key experimental and theoretical questions, from the underlying symmetries at the smallest scales to the particle nature of dark matter. A new particle, the QCD axion, could shed light on both fronts and throw open a window on energies otherwise inaccessible to experiment. However, the axion presents a challenging search target: it interacts very feebly, making it difficult to produce and detect, and its minute mass ranges across many orders of magnitude, requiring a host of approaches. I will outline directions that leverage novel effects in the extreme environments of the early universe, neutron stars, and black holes to test new axion and other light particle parameter space. I will then focus on my dark matter proposals based on dielectric metamaterials and describe how volume-filling geometries such as disordered media can counterintuitively act as efficient axion-to-photon conversion targets.