Dave I would say that primordial black holes were called Quasars.
Very old and very far away.

Massive black holes come in many varieties like super massive black holes and some even larger.

That black holes behave like elementary particles is paramount to understanding what they are and how our universe is governed by their existence.

In the early universe the relative rareness of black holes compared to a universe today where the number of black holes continues to grow exponentially is effecting the fundamental behavior and evolutionary accelerating growth of our universe.

It is my hypothesis that this increase in black holes creates a response of space itself to accelerate its expansion. This balancing act phenomena is what some people call the mysterious dark energy.
Sooner or later the energy limits of trying to seek stability is surpassed and a entropy event will occur.

This concept was long in the making but I feel confident that it ranks with the other explanation of dark energy which is "haven't the foggiest".