Based on dynamical estimates of the mass density of aggregated systems, at least 90% of the matter in the Universe is unseen. Furthermore, it is unlikely that more than a small fraction of it is ordinary (baryonic) matter. A particle relic from the Big Bang is an attractive solution, with a very light axion being one of two leading candidates. Due to its extremely weak couplings to anything, the axion was long thought to be 'invisible'. In 1983 however, Pierre Sikivie showed how axions could be detected by their resonant conversion to photons, in a microwave cavity permeated by a strong magnetic field. We have built and operated a large-scale experiment which for the first time has achieved cosmological sensitivity for dark matter axions constituting our galactic halo. The development of gigahertz SQUID amplifiers for the axion experiment will be briefly described; this is a technology breakthrough enabling an upgrade to a truly definitive experiment.