We show that the presence of a \rho \sim r ^ { -3 / 2 } dark matter overdensity can be robustly predicted at the center of any galaxy old enough to have grown a power-law density cusp in the stars via the Bahcall-Wolf mechanism .
Using both Fokker-Planck and direct N -body integrations , we demonstrate collisional generation of these dark matter “ crests ” ( Collisionally REgenerated STtructures ) even in the extreme case that the density of both stars and dark matter were previously lowered by slingshot ejection from a binary supermassive black hole .
The time scale for collisional growth of the crest is approximately the two-body relaxation time as defined by the stars , which is \lesssim 10 Gyr at the centers of stellar spheroids with luminosities L \lesssim 10 ^ { 9.5 } L _ { \odot } , including the bulge of the Milky Way .
The presence of crests can robustly be predicted in such galaxies , unlike the steeper enhancements , called “ spikes ” , produced by the adiabatic growth of black holes .
We discuss special cases where the prospects for detecting dark matter annihilations from the centers of galaxy haloes are significantly affected by the formation of crests .