1. Institute for Theoretical Physics, University of Zurich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
2. Present address: Department of Astronomy and Astrophysics, University of California, 1156 High Street, Santa Cruz, California 95064, USA

Correspondence to: B. Moore¹ Correspondence and requests for materials should be addressed to B.M. (Email: moore@physik.unizh.ch).

The Universe was nearly smooth and homogeneous before a redshift of z = 100, about 20 million years after the Big Bang¹. 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^{2, 3, 4}, the determination of which is one of the biggest challenges in present-day science^{5, 6, 7}. 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 10¹⁵ 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 -rays (from particle–particle annihilation).

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