  High energy collisions of particles may have created tiny black holes in the
early Universe, which might leave stable remnants instead of fully evaporating
as a result of Hawking radiation. If the reheating temperature was sufficiently
close to the fundamental gravity scale, which can be different from the usual
Planck scale depending of the presence and properties of spatial
extra-dimensions, the formation rate could have been sufficiently high and
hence such remnants could account for the entire cold dark matter of the
Universe.
