keV-mass dark matter candidates and constraints

The cold dark matter cosmological model has been extremely successful in explaining cosmic structure on large scales, but has ongoing challenges from observations that probe small-scale structures. Warm dark matter (WDM) provides a compelling alternative that help resolve such issues whilst maintaing the successes on large scales. After overviewing motivations for going beyond WIMP-based dark matter, I will review candidates of keV-mass WDM particles and their production mechanisms in the early Universe. I will then discuss their cosmological implications and explore the variety of constraints coming from structure as well as X-ray observations.