Here's why:

* Newton's Law of Universal Gravitation: This law states that every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

* Inverse Square Law: The "inversely proportional to the square of the distance" part is crucial. This means that as the distance between two objects increases, the gravitational force between them decreases rapidly, but it *never* reaches zero, even at an infinite distance.

Practical Considerations:

While theoretically infinite, the gravitational force becomes incredibly weak at large distances. At some point, the force is so weak that it's practically negligible. This is why we don't feel the gravitational pull of distant stars or galaxies, even though they have immense mass.

Example:

* The Earth's gravity pulls on the Moon, even though they are 384,400 km apart.

* The Sun's gravity holds all the planets in our solar system, even though the farthest planet, Neptune, is 4.5 billion km away.

In summary:

* Gravitational force extends theoretically infinitely.

* In practice, the force becomes extremely weak at large distances and can be considered negligible.