
Ultralight scalar dark matter with mass at or below the eV scale and above 10^{-22} eV and pressure from repulsive self-interaction could form a Bose-Einstein condensate in the early Universe and maybe in galaxies as well. It has been suggested before that this scenario may provide a possible solution to the cusp/core problem and some specific model may even explain MOND phenomenology. In this talk, I will discuss some challenges facing this scenario from the particle physics model building point of view. I will also discuss some cosmological and astrophysical constraints on ultralight repulsive dark matter in terms of a parametrization motivated by particle physics considerations.