Final answer:
Gravitational forces act at a distance because of the existence of a gravitational field that extends from any mass. Despite being the weakest fundamental force, it is dominant in large-scale astronomical systems and is always attractive.
Step-by-step explanation:
Gravitational forces act on objects without being in contact with them because the gravitational field extends out from a mass. This field is a concept to explain how objects exert force on each other over a distance. Although gravity is by far the weakest of the four fundamental forces, it is significant on a large scale where the gravitational pull of a massive body, like the Earth or the Sun, can dominate. Gravitational force is always attractive and acts between any two objects with mass, which is why we experience weight and why planets revolve around stars.