The principle that explains the conservation of momentum in closed systems is known as the law of conservation of momentum. This law states that in the absence of external forces, the total momentum of a closed system remains constant over time. Momentum is defined as the product of an object's mass and velocity. In collisions or interactions between objects, momentum can be transferred, yet the total momentum before and after the event will remain unchanged. This principle is fundamental in physics and can be observed in systems ranging from subatomic particles to planetary collisions.