Cosmic CO and [C II] backgrounds and the fuelling of star formation over 12 Gyr

Molecular gas, modest in mass yet pivotal within the cosmic inventory, regulates baryon cycling as the immediate fuel for star formation. Across most of cosmic history, its reservoir has remained elusive, with only the tip of the iceberg revealed by luminous CO-emitting galaxies. Here we report the detection of the mean cosmic CO background across its rotational ladder at 7σ, together with ionized carbon ([C II]) at 3σ, over 0 < z < 4.2. This uses tomographic clustering of diffuse broadband intensities with reference galaxies, directly probing aggregate emission in the cosmic web. From CO(1–0) we infer the total molecular gas density, _H₂ , finding it about twice that resolved in galaxy surveys. The global depletion time is ~1 Gyr, shorter than the Hubble time, requiring sustained inflow. CO excitation is linked to star-formation surface density and, with depletion time, yields a super-linear Kennicutt–Schmidt law that appears universal. Together these results establish a global picture of galaxy growth fuelled by a larger, short-lived molecular reservoir. The CO and [C II] detections mark a turning point for line-intensity mapping, replacing forecasts with empirical line strengths and defining sensitivity requirements for upcoming three-dimensional experiments poised to open new windows on galaxy formation and cosmology. Detections of the mean cosmic CO and [C II] line backgrounds over 12 billion years reveal a large, rapidly replenished molecular gas reservoir that fuels star formation and set benchmarks for line-intensity mapping.