The cD galaxy Messier 87 is the biggest galaxy of the local Universe. High-quality photometric and spectroscopic measurements yield an accurate mass distribution that shows the presence of both a stellar spheroid and of a spherical dark matter halo, extended out to 1.3 Mpc. The mass of the stellar spheroid 20 times the mass of the Milky Way galaxy and its dark matter halo is 100 times bigger. Remarkably, the dark matter halo shows a very large central region with an about constant density.