The two particles appear to be pulled together by gravitation. This is how gravity works. Thus, the theory of general relativity is in fact the theory of gravitation. Space-time curved by a massive ball. The tiny ball also bends space-time, but the curvature is negligible compared to that made by the massive ball.
Gravity works in a similar way.
Although the other three fundamental forces play vital roles in a star, they are irrelevant when it comes to the scale of galaxies. This is because weak and strong nuclear forces are only short range forces. Their influence does not go beyond the nuclei of atoms. Electromagnetic force is a long range force like gravity. However, unlike gravity, it not only attracts but also repels depending on what charges are involved.