Abstract

In cold dark matter cosmological models1,2, structures form and grow through the merging of smaller units3. Numerical simulations have shown that such merging is incomplete; the inner cores of haloes survive and orbit as ‘subhaloes’ within their hosts4,5. Here we report a simulation that resolves such substructure even in the very inner regions of the Galactic halo. We find hundreds of very
concentrated dark matter clumps surviving near the solar circle, as well as numerous cold streams. The simulation also reveals the fractal nature of dark matter clustering: isolated haloes and subhaloes contain the same relative amount of substructure and both have cusped inner density profiles. The inner mass and phasespace densities of subhaloes match those of recently discovered faint, dark-matter-dominated dwarf satellite galaxies6–8, and the
overall amount of substructure can explain the anomalous flux ratios seen in strong gravitational lenses9,10. Subhaloes boost c-ray production from dark matter annihilation by factors of 4 to 15 relative to smooth galactic models. Local cosmic ray production is also enhanced, typically by a factor of 1.4 but by a factor of
more than 10 in one per cent of locations lying sufficiently close to a large subhalo. (These estimates assume that the gravitational effects of baryons on dark matter substructure are small.)