Answer: Red dwarfs
Step-by-step explanation:
Red dwarfs are thought to be the coolest star out of the rest. They are also the smallest. Because they are so cool, their luminosity is low as well on account of the fact that relatively speaking, the fusion taking place on their surface is little.
Red dwarfs are the most numerous star in the universe and the most numerous in the Milky Way galaxy as well. We just can't see them with out naked eyes because of their low luminosities.