In multi-component dark matter scenarios, dynamics of a sub-component dark matter has been often ignored. In this talk, I will be pointing out the hidden roles of a sub-component dark matter in the scenarios where the dominant relics interact with the Standard Model sector only indirectly through the sub-component.
Firstly, I will briefly discuss the multi-component boosted dark matter (BDM) which uses the sub-component in probing the dominant relic and opens up new prospects in probing the dark sector in unique ways in various dark matter and neutrino experiments. Secondly, distinctive cosmological dynamics of a sub-component dark matter will be discussed. The thermal evolution of the sub-component is significantly affected by the sizable self-scattering and the required annihilation cross section of the sub-component sharply increases as we consider a smaller relative abundance fraction among the dark-matter species. Therefore, contrary to a naive expectation, it can be easier to detect the sub-component with smaller abundance fractions in direct/indirect-detection experiments and cosmological observations.