Can reionization shadows reveal what dark matter really is?

Warm dark matter suppresses small galaxies, but most constraints come from tiny-scale probes. This work uses a new angle: the aftermath of reionization (when the universe's hydrogen got ionized) imprints a distinctive pattern in the intergalactic medium that depends sensitively on whether dark matter is warm or cold. Combining Lyα forest data with 21 cm observations could push the lower bound on warm dark matter mass from ~10 keV to 39 keV—a dramatic gain from one large-scale effect.