  The recent detection of a 3.5 keV X-ray line from the centres of galaxies and
clusters by Bulbul et al. (2014a) and Boyarsky et al. (2014a) has been
interpreted as emission from the decay of 7 keV sterile neutrinos which could
make up the (warm) dark matter (WDM). As part of the COpernicus COmplexio
(COCO) programme, we investigate the properties of dark matter haloes formed in
a high-resolution cosmological $N$-body simulation from initial conditions
similar to those expected in a universe in which the dark matter consists of 7
keV sterile neutrinos. This simulation and its cold dark matter (CDM)
counterpart have $\sim13.4$bn particles, each of mass $\sim 10^5\, h^{-1}
M_\odot$, providing detailed information about halo structure and evolution
down to dwarf galaxy mass scales. Non-linear structure formation on small
scales ($M_{200}\, \leq\, 2 \times 10^9\,h^{-1}\,M_\odot$) begins slightly
later in COCO-Warm than in COCO-Cold. The halo mass function at the present day
in the WDM model begins to drop below its CDM counterpart at a mass $\sim 2
\times 10^{9}\,h^{-1}\,M_\odot$ and declines very rapidly towards lower masses
so that there are five times fewer haloes of mass $M_{200}=
10^{8}\,h^{-1}\,M_\odot$ in COCO-Warm than in COCO-Cold. Halo concentrations on
dwarf galaxy scales are correspondingly smaller in COCO-Warm, and we provide a
simple functional form that describes its evolution with redshift. The shapes
of haloes are similar in the two cases, but the smallest haloes in COCO-Warm
rotate slightly more slowly than their CDM counterparts.
