  In the standard model of cosmic structure formation, dark matter haloes form
by gravitational instability. The process is hierarchical: smaller systems
collapse earlier, and later merge to form larger haloes. The galaxy clusters,
hosted by the largest dark matter haloes, are at the top of this hierarchy
representing the largest as well as the last structures formed in the universe,
while the smaller and first haloes are those Earth-sized dark subhaloes which
have been both predicted by theoretical considerations and found in numerical
simulations, though it does not exist any observational hints of their
existence. The probability that a halo of mass $m$ at redshift $z$ will be part
of a larger halo of mass $M$ at the present time can be described in the frame
of the extended Press & Schecter theory making use of the progenitor
(conditional) mass function. Using the progenitor mass function we calculate
analytically, at redshift zero, the distribution of subhaloes in mass,
formation epoch and rarity of the peak of the density field at the formation
epoch. That is done for a Milky Way-size system, assuming both a spherical and
an ellipsoidal collapse model. Our calculation assumes that small progenitors
do not lose mass due to dynamical processes after entering the parent halo, and
that they do not interact with other subhaloes. For a $\mathrm{\Lambda}$CDM
power spectrum we obtain a subhalo mass function $\mathrm{d}n/\mathrm{d}m$
proportional to $m^{- \alpha}$ with a model-independent $\alpha \sim 2$.
Assuming the dark matter is a weakly interacting massive particle, the inferred
distributions is used to test the feasibility of an indirect detection in the
$\gamma$-rays energy band of such a population of subhaloes with a GLAST-like
satellite.
