  Condensation of bosons causes spectacular phenomena such as superfluidity or
superconductivity. Understanding the nature of the condensed particles is
crucial for active control of such quantum phases. Fascinating possibilities
emerge from condensates of light-matter coupled excitations, such as exciton
polaritons, photons hybridized with hydrogen-like bound electron-hole pairs. So
far, only the photon component has been resolved, while even the mere existence
of excitons in the condensed regime has been challenged. Here we trace the
matter component of polariton condensates by monitoring intra-excitonic
terahertz transitions. We study how a reservoir of optically dark excitons
forms and feeds the degenerate state. Unlike atomic gases, the atom-like
transition in excitons is dramatically renormalized upon macroscopic ground
state population. Our results establish fundamental differences between
polariton condensation and photon lasing and open possibilities for coherent
control of condensates.
