"An apple fell and changed the course of physics. I pull everything toward everything else. I am the weakest of the fundamental forces yet I hold galaxies together. What am I?"

Gravity

The story of Newton observing a falling apple and conceiving his theory of gravity is possibly apocryphal but culturally important. Gravity is the weakest of the four fundamental forces — far weaker than electromagnetism or nuclear forces — yet because it acts over infinite range and is always attractive, it structures the entire universe.