The QCD axion simultaneously explains the smallness of neutron EDM and provides a compelling candidate for Dark Matter. In this talk, I will first highlight synergies between CMB (cosmic microwave background) cosmology and the search for QCD axion. I will then focus on the "axion-haloscope", which uses the inverse Primakoff conversion of axions in a resonant cavity under an external magnetic field. A conventionalhaloscope’s sensitivity decreases precipitously in frequency, making it difficult to probe the axion mass range in the cm-wavelengths (or >~ 10ueV). I will describe a novel experimental approach that can enhance the scan rate by several orders of magnitude using space-filling thin-shell cavities, including the promises and challenges. I will conclude by providing an outlook, and an update on our experimental effort to demonstrate a prototype.