  Primordial dark matter (DM) haloes are the smallest gravitationally bound DM
structures from which the first stars, black holes, and galaxies form and grow
in the early universe. However, their structures are sensitive to the free
streaming scale of DM, which in turn depends on the nature of DM particles. In
this work, we test the hypothesis that the slope of the central cusps in
primordial DM haloes near the free streaming scale depends on the nature of
merging process. By combining and analysing data from a cosmological simulation
with the cutoff in the small-scale matter power spectrum as well as a suite of
controlled, high-resolution simulations of binary mergers, we find that (1) the
primordial DM haloes form preferentially through major mergers in radial
orbits; (2) their central DM density profile is more susceptible to a merging
process compared to that of galaxy and cluster-size DM haloes; (3) consecutive
major mergers drive the central density slope to approach the {\it universal}
form characterized by the NFW profile, which is shown to be robust to the
impacts of mergers and serves an attractor solution for the density structure
of DM haloes. Our work highlights the importance of dynamical processes on the
structure formation during the Dark Ages.
