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In the ΛCDM paradigm, the nonlinear growth of DM structure is a well-posed problem where both the initial conditions and the evolution equations are known (at least when the effects of the baryons can be neglected).

The Aquarius Project [54] is a Virgo Consortium program to carry out high-resolution DM simulations of Milky-Way–sized halos in the ΛCDM cosmology. This project seeks clues to the formation of galaxies and to the nature of the dark matter by designing strategies for exploring the formation of our Galaxy and its luminous and dark satellites.

The galaxy population on scales from 50 kpc to the size of the observable Universe has been predicted by hierarchical ΛCDM scenarios, and compared directly with a wide array of observations. So far, the ΛCDM paradigm has passed these tests successfully, particularly those that consider the large-scale matter distribution and has led to the discovery of a universal internal structure for DM halos. As was noted in Sec. 12, the observed structure of galaxies, clusters and superclusters, as illustrated by Fig. 16, could not have formed in a baryonic medium devoid of dark matter.

Figure 16

Figure 16. The left panel shows the projected dark matter density at z = 0 in a slice of thickness 13.7 Mpc through the full box (137 Mpc on a side) of the 9003 parent simulation. The right panel show this halo resimulated at a different numerical resolution. The image brightness is proportional to the logarithm of the squared DM density projected along the line-of-sight. The circles mark r50, the radius within which the mean density is 50 times the background density. From V. Springel & al. [54]

Given this success, it is important to test ΛCDM predictions also on smaller scales, not least because these are sensitive to the nature of the dark matter. Indeed, a number of serious challenges to the paradigm have emerged on the scale of individual galaxies and their central structure. In particular, the abundance of small DM subhalos predicted within CDM halos is much larger than the number of known satellite galaxies surrounding the Milky Way (M. Boylan-Konchin & al. [48] and references therein).

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