The cuspy-core problem is one of the main challenges of the cold dark matter paradigm on small scales: the density of a dark matter halo is predicted to rise rapidly toward the center as ρ(r) ~ r-(1.0–1.5), while such a cuspy prole has not been clearly observed. We have carried out the spatially resolved mapping of gas dynamics toward a nearby ultra-diuse galaxy (UDG), AGC 242019. The derived rotation curve of dark matter is well fitted by the cuspy profile as described by the Navarro-Frenk-White (NFW) model, while the cored profiles including both the pseudo-isothermal and Burkert models are excluded at a high confidence. With the generalized NFW formula, the halo is further shown to have a central density slope of -0.95+-0.06. AGC 242019 thus demonstrates the validity of the cold dark matter paradigm on sub-galactic scales. I will also present its quantitative constraints on the alternatives of cold dark matter (fuzzy, self-interacing & warm) and its implication for the feedback model and formation of ultra-diffuse galaxies.