It's not difficult to construct a situation where dipole attraction would be unstable, but gravity isn't. E.g. an odd number of point particles evenly spaced in a circle.

(Real answer: plausible-sounding explanations are ten a penny. Which is why scientific theories claiming Y is an example of X generally need to either (a) mathematically derive the governing equations of Y starting from those of X, or else (b) make a testable prediction about Y that would not be true if Y was not an example of X).

Current theories predict that neutrons will attract each other. It's plausible that some of the things we see in space are composed primarily of neutron, and they seems to have an attractive force proportional to their mass.

Finally, black holes certainly seem to have gravity, and they have nothing outside the event horizon.

Have you considered trying to make your theory apply in these cases?

Gravity has fundamental differences from the other force, one of the key ones being related the general relativity principles. Gravity causes a distortion in space-time and it is why we have to adjust clocks on satellite in order to match the clocks not in orbit.

Gravity has fundamental principle that makes it different than electromagnetic forces.