  The Universe was nearly smooth and homogeneous before a redshift of z = 100,
about 20 million years after the Big Bang. After this epoch, the tiny
fluctuations imprinted upon the matter distribution during the initial
expansion began to collapse because of gravity. The properties of these
fluctuations depend on the unknown nature of dark matter, the determination of
which is one of the biggest challenges in present-day science. Here we report
supercomputer simulations of the concordance cosmological model, which assumes
neutralino dark matter (at present the preferred candidate), and find that the
first objects to form are numerous Earth-mass dark-matter haloes about as large
as the Solar System. They are stable against gravitational disruption, even
within the central regions of the Milky Way. We expect over 10^15 to survive
within the Galactic halo, with one passing through the Solar System every few
thousand years. The nearest structures should be among the brightest sources of
gamma-rays (from particle-particle annihilation).
