I agree with your suspicion that they may be hollow, and that they expand and contract at certain intervals.
The duration of the expansion/contraction cycle, I think, would depend on the amount of mass that surrounds the hole.
For instance, the cycle duration for the entire universe would be very long, as you point out, but then the galaxies might have a faster expansion/contraction cycle since they are smaller and have less mass to accelerate and decelerate.

To over simplify the whole idea, the hollow hole of the black hole draws the mass towards its center, and when the mass totally fills the hole, then the kinetic energy of the incoming mass causes an explosion, that explosion then causes the mass to expand until the black hole is powerful enough to draw the mass back in on its self, repeating the cycle.

Protons and electrons may also be such micro mini black hole/mass combinations.
At least that is what I think. The formula that I have presented here in the past seems to agree with such assumptions.

Johannes