The Nature of Light Dark Matter

For over seventy years we have observed evidence for the presence of dark matter, now clearly shown to be non‐baryonic. The microwave background fluctuations show that it is about 0.2 of all density of matter and energy, while baryonic matter is about 0.04; the rest is labelled dark energy, and not even close to an understanding. The sum is just what is required to make the universe geometrically flat. In this overview we discuss one of the attempts to explain the nature of dark matter, light dark matter, also called sterile neutrinos, or right‐handed neutrinos, or more generally warm dark matter. Many of the predictions for cold dark matter and warm dark matter are the same, most of modifications are in small scales, in the temporal evolution of structure, and in some of the shapes of structure. We are now convinced that these warm dark matter particles have to be subthermal at injection to get the right free‐streaming length in structure formation. Galaxies, their structure as well as their distribution pose a number of tests and riddles, not all of which have yet been passed or solved. We outline and summarize the latest evolution in these discussions, centering now on galaxies, their structure and evolution, as well as large scale distribution. Warm dark matter has survived many critical tests so far, and is clearly worth exploring in much more detail.