Dark energy does not exist. The illusion of dark energy is caused by the difference in the universe's expansion rate at the outskirts of the visible universe.

At some far point in deep space, the universe's expansion rate slows down to a trickle. Massive amounts of matter accumulate in that outer layer to the point that massive black holes are formed.

This colossal "wall" of black holes around our visible universe creates a strong gravitational field, tugging on all matter in the visible universe.

As anything being pulled by a gravitational field, astronomical objects move toward the gravitational source at increasing speeds, thus giving the illusion of what we refer to as dark energy.