Dark matter haloes formed in ∧CDM cosmologies exhibit a characteristic dependence of density on distance from the centre (Chapter 2). Early studies [721; 1501] established ρDM ∼ r−3 or r−4 at large radii and ρDM ∼ r−1 inside the virial radius. On still smaller scales, the form of ρDM(r) was little more than an ansatz since the relevant scales were barely resolved in the N-body simulations. A debate ensued as to whether the profiles were indeed universal and, if so, what power of the radius described the dark matter density in the limit r → 0. Subsequent studies found central profiles both steeper [666; 940; 1247; 1471] and shallower [1431; 1503; 1504] than r−1.

The focus of this chapter is the dark matter distribution on sub-parsec scales. At these radii, the gravitational force in many galaxies is known to be dominated by the observed baryonic components (stellar bulge, nuclear star cluster) and by the supermassive black hole. Dark matter densities at these radii are barely constrained observationally; however, they could plausibly be orders of magnitude higher than the local value at the solar circle (∼10−2M⊙ pc−3), owing both to the special location at the centre of the halo and to interactions between dark matter and baryons during and after formation of the galaxy. High dark matter densities make the centres of galaxies preferred targets for indirect detection studies, in which secondary particles and photons from the annihilation or decay of supersymmetric dark matter particles are detected on the Earth (Chapter 24).