The cold dark matter cosmological paradigm has been extremely successful in explaining
cosmic structure on large scales. Yet, it has constantly been challenged by observations
that probe small-scale structures. I will first introduce these challenges, and discuss
how warm dark matter provides an alternative that help resolve such small-scale issues
whilst maintaing the successes on large scales. From the particle perspective, I will
focus on sterile neutrinos, one of th leading warm dark matter candidates. Finally,
anomalous 3.5 keV lines have recently been reported, which may be interpreted as sterile
neutrino dark matter decay signals. I will discuss implications of this signal in the
context of sterile neutrino cosmology.