The force of gravity is considered to be constant near the surface of the Earth, but it is not actually constant throughout the universe. The force of gravity is determined by the mass of the objects involved and the distance between them. According to Newton's law of universal gravitation, the force of gravity between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Therefore, the force of gravity is not constant because:

1. Mass of objects: The mass of objects can change, and this can affect the force of gravity. For example, if one object gains or loses mass, the force of gravity between the two objects will change.

2. Distance between objects: The distance between objects can change, and this can affect the force of gravity. For example, if two objects move closer together, the force of gravity between them will increase. Conversely, if two objects move further apart, the force of gravity between them will decrease.

3. Gravitational fields: The presence of other objects with significant mass can create gravitational fields that influence the force of gravity. For instance, the presence of a planet or a large celestial body can create a gravitational field that alters the force of gravity experienced by objects within its vicinity.

The force of gravity is often considered constant near the Earth's surface because, for everyday objects and distances near the surface, the variations in mass and distance are relatively small, resulting in a negligible change in the force of gravity. However, on a cosmic scale, the force of gravity can vary significantly based on the masses and distances involved.