The concept in physics explaining why a ball thrown into the air eventually falls back down is known as gravity. Gravity is an attractive force that pulls objects toward one another, allowing celestial bodies like Earth to exert a gravitational pull on objects on its surface. As a ball ascends, it slows due to gravity until it reaches its peak height. Afterward, the force of gravity accelerates the ball downward, returning it to the ground. This fundamental principle of gravity is essential in understanding motion and forces in the universe.