We introduce an innovative design of planar plasmonic nanogap antenna arrays and demonstrate its potential to study the spatiotemporal organization of mimetic biological membranes at the nanoscale. We exploit our novel nanogap antenna platform with different nanogap sizes (10-45 nm) combined with fluorescence correlation spectroscopy to reveal the existence of nanoscopic domains in mimetic biological membranes. Our approach takes advantage of the highly enhanced and confined excitation light provided by the antennas together with their extreme planarity to investigate membrane regions as small as 10 nm in size with microsecond temporal resolution. We first demonstrate the ultra-high confinement of photonic antennas on biological membranes. Moreover, we show that cholesterol slows down the diffusion of individual fluorescent molecules embedded in the lipid bilayer, consistent with the formation of nanoscopic domains enriched by cholesterol. Incorporation of hyaluronic acid (HA) to the ternary lipid mixture further slows down molecular diffusion, suggesting a synergistic effect of cholesterol and HA on the dynamic partitioning of mimetic biological membranes.