The universe stopped making stars and ΛCDM does not know why then. The cosmic star formation rate density rose through the first three billion years, peaked at z of about 2, cosmic noon, and has fallen by roughly a factor of ten since: a cliff in cosmic history, sharp in its onset and global across environments. The decline is not for lack of fuel: gas accretion onto halos continues, the molecular reservoirs of galaxies at z of 1 to 2 remain substantial, and the dark matter halos hosting star formation keep growing through the entire decline. The model must therefore arrange for star formation efficiency itself to fall everywhere, roughly simultaneously, by an order of magnitude.
The standard toolkit assigns the cliff to feedback: AGN heating quenches massive galaxies, supernovae regulate small ones, and environmental processes strip satellites. Each mechanism is real, but their sum is a patchwork tuned per simulation to reproduce the decline after the fact, and they face the timing problem: feedback strength tracks local conditions, not cosmic epoch, so a globally synchronized downturn at z of 2 across field and cluster, massive and small, requires the patchwork to conspire. Halo-quenching thresholds and declining merger-driven triggering help in simulations whose star formation histories nevertheless systematically misplace or soften the peak unless recalibrated against it.
The standing is a structural puzzle hiding in plain sight: the single most prominent feature of cosmic star formation history is reproduced by construction rather than predicted, and its global synchronization remains unexplained. Surveys mapping gas, star formation, and environment jointly through the peak epoch (ALMA deep fields, MOONS, upcoming wide IFU programs) are quantifying exactly how uniform the downturn is, sharpening the question of what global clock turned the universe off.