Extreme hydrodynamics in Dirac fluid

Near quantum criticality, charge transport can exhibit universal behavior independent of microscopic details. Dirac fluids---strongly interacting electron-hole plasmas in graphene---are predicted to realize such universality, yet direct and accurate experimental quantification has been hindered by phonon and defect scattering that obscure intrinsic hydrodynamic behavior. By transiently heating electrons in high-quality hBN-encapsulated monolayer graphene to several thousand kelvin, we access a pristine Dirac fluid deep in the extreme hydrodynamic regime, where disorder and phonon effect are negligible. Using nano-terahertz spacetime mapping and plasmonic worldline metrology, we directly visualize the electrodynamics of this quantum-critical fluid and observe a pronounced slowdown of plasmon propagation driven by strong electron-hole collisions. In this limit, both plasmon velocity and lifetime saturate to universal values determined by fundamental constants. Immune to extrinsic scattering, these measurements reveal the universal conductance of a pristine Dirac fluid and demonstrate a powerful new approach for probing extreme hydrodynamic phenomena and nonequilibrium quantum matter on its native space and time scale.