Dark Matter Free-Streaming: Cold vs Warm vs Hot

The mass of the dark-matter particle decides how far it can travel before gravity gets a chance to organise it into structure — this is the free-streaming problem at the heart of cosmology's cold-vs-warm-vs-hot dark matter debate. This simulator seeds a comoving volume with cluster-scale and dwarf-scale density perturbations, computes the Maxwell–Boltzmann thermal velocity and thermal-relic free-streaming length for whatever particle mass you dial in (from an eV-scale neutrino to a hundred-GeV WIMP), and evolves the particles under toggleable self-gravity plus a free-streaming random walk sized directly from that length. Small clumps dissolve first as the particle gets lighter, then large ones too — visually reproducing why light, fast-moving dark matter erases small-scale structure while cold, heavy dark matter preserves it down to sub-galactic scales.