Direct detection of cool molecular gas in a star-forming galaxy at z=7.31

We investigate the molecular gas content and interstellar medium (ISM) conditions of REBELS-25, a massive, star-forming galaxy at (Formula presented). Deep VLA (Very Large Array) Q-band and Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 observations reveal CO(3−2) and CO(7−6) emission (both at (Formula presented)), and provide an upper limit on [C i](2−1). From the CMB (cosmic microwave background)-corrected CO(3−2) flux – representing the highest redshift detection of a low-J CO transition to date – we derive a molecular gas mass of (Formula presented), directly confirming the presence of a very massive gas reservoir only (Formula presented) Myr after the big bang. This implies an extreme gas fraction of (Formula presented), a gas-to-dust ratio of (Formula presented), and a depletion time-scale of (Formula presented)  Gyr, broadly consistent with extrapolated scaling relations for main-sequence galaxies at lower redshift. Using the radiative transfer code tuner, we self-consistently model CO and dust continuum emission in the context of the significant CMB background, constraining ISM properties and recovering (Formula presented), independent of assumptions about (Formula presented) and (Formula presented). We further discuss the use of alternative molecular gas tracers at early epochs. Combining CO and [C ii] measurements, we infer an empirical [C ii]-to-H(Formula presented) conversion factor of (Formula presented) (Formula presented) /(Formula presented), suggesting [C ii] remains a viable molecular gas tracer in the epoch of reionization. These results demonstrate the detectability of low-J CO emission even at (Formula presented), paving the way for next-generation facilities, and provide critical insights into the rapid mass assembly of galaxies during the first billion years of cosmic history.