Black holes have been observed (insofar as anything in astrophysics is observed) at the center of galaxies and do not suffice to explain why they hold together. So they're no longer a postulate, and they do not serve the purpose described.
Dark matter and dark energy are postulated as such an explanation.
I hope your kidding, but if your both serious and not trolling:
Like charges repel and opposite charges attract which prevents large scale accumulation of charge in even a tiny area by cosmic scales. Gravity while locally weak does not have these issues and you can keep dumping more mass into the same area more or less indefinitely.
Dumping more and more mass into a black hole until it holds a whole universe is, apparently, not enough. But, hey, isn't it lucky that there is now also an unobservable dark matter and energy and just enough of it to make the 'gravity only' sums come out right? Hmmmm.
Does this really sound more plausible to you than that widely separated bodies in vacuum, large and small, can hold charge?
Keep reading your high-school textbook on physics and be sure to note the equations on calculating the force of an electrostatic charge over very large distances.
If you rubbed the entire galaxy with a cosmic-sized cat you still wouldn't have the effects you're postulating.
Electrostatic force is much much stronger than gravity. It is known to be ubiquitous both on micro (sub-atomic) scale and on macro (cosmic) scale.
Once you acknowledge its existence, it is no longer necessary to postulate black holes in the centres of galaxies to hold them together.