I cannot be seen, cannot be felt, cannot be hard, cannot be smelt. I lie behind stars and under hills. I fill empty holes. I come first and follow after, end life and kill laughter. Who am I?

The enigmatic subject of this riddle is darkness, a concept that is not a tangible entity but rather the absence of light. This is why it cannot be seen, felt, heard, or smelt; it lacks physical properties that our senses can detect. Darkness exists in the vast expanse of space, truly lying "behind stars," and pervades subterranean environments, existing "under hills" where light cannot penetrate.

The riddle continues by stating that it "fills empty holes." This refers to how darkness naturally occupies any space that is not illuminated. Wherever light is absent, darkness is inherently present. Furthermore, the lines "I come first and follow after" aptly describe the cyclical nature of day and night. Darkness precedes dawn and returns after dusk, marking the beginning and end of each day, a constant presence that light temporarily displaces.

Lastly, the phrases "end life and kill laughter" tap into the profound metaphorical and psychological impact darkness has on human experience. Culturally, darkness is often associated with the unknown, fear, sorrow, or despair. It can symbolize ignorance or a lack of understanding, which can metaphorically diminish joy or bring an end to vibrant life. This emotional response demonstrates how darkness transcends its scientific definition to deeply influence our perceptions and feelings.