  We explore the cosmological halo-to-halo scatter of the distribution of mass
within dark matter halos utilizing a well-resolved statistical sample of
clusters from the cosmological Millennium simulation. We find that at any
radius, the spherically-averaged dark matter density of a halo (corresponding
to the "smooth-component") and its logarithmic slope are well-described by a
Gaussian probability distribution. At small radii (within the scale radius),
the density distribution is fully determined by the measured Gaussian
distribution in halo concentrations. The variance in the radial distribution of
mass in dark matter halos is important for the interpretation of direct and
indirect dark matter detection efforts. The scatter in mass profiles imparts
approximately a 25 percent cosmological uncertainty in the dark matter density
at the Solar neighborhood and a factor of ~3 uncertainty in the expected
Galactic dark matter annihilation flux. The aggregate effect of halo-to-halo
profile scatter leads to a small (few percent) enhancement in dark matter
annihilation background if the Gaussian concentration distribution holds for
all halo masses versus a 10 percent enhancement under the assumption of a
log-normal concentration distribution. The Gaussian nature of the cluster
profile scatter implies that the technique of "stacking" halos to improve
signal to noise should not suffer from bias.
