Final answer:
Gravity is considered a weak force because its strength depends on the mass of the interacting objects. Repulsive gravitons are hypothetical particles that have not been proven to exist and do not prevent singularities from forming.
Step-by-step explanation:
Gravity is considered a weak force compared to the other three fundamental forces because its strength depends on the mass of the interacting objects. The gravitational force between two objects is given by Newton's law of universal gravitation, which states that the force is proportional to the product of their masses and inversely proportional to the square of the distance between them. So, even though the force of gravity is weak at the scale of elementary particles, it becomes noticeable and significant when interacting with large masses like planets and stars.
As for repulsive gravitons, they are hypothetical particles that have not been observed or proven to exist. In the currently accepted theory of gravity, general relativity, gravity is only attractive and does not have a repulsive component. Singularities, such as those found in black holes, are not prevented by repulsive gravitons because they arise from the extreme curvature of spacetime caused by mass, not from repulsive gravitational forces.