The first robust evidence showing a dark matter density spike around the supermassive black hole in OJ 287

Black hole dynamics suggests that dark matter would re-distribute near a supermassive black hole to form a density spike. However, no direct evidence of dark matter density spike around a supermassive black hole has been identified. In this letter, we present the first robust evidence showing a dark matter density spike around a supermassive black hole. We revisit the data of the well-known supermassive black hole binary OJ 287 and show that the inclusion of the dynamical friction due to a dark matter density spike around the supermassive black hole can satisfactorily account for the observed orbital decay rate. The derived spike index $\gamma_{\rm sp}=2.351^{+0.032}_{-0.045}$ gives an excellent agreement with the value $\gamma_{\rm sp}=2.333$ predicted by the benchmark model assuming an adiabatically growing supermassive black hole. This provides a strong verification of the canonical theory suggested two decades ago modeling the gravitational interaction between collisionless dark matter and supermassive black holes.