Dark energy and virtual particles

If space time is accelerating away from itself everywhere due to dark energy,

In general , given asymmetric mass distributions, gravitational waves are generated from the space time distortions.

could this acceleration be the cause of virtual particles,

Assuming quantization of gravity and the reality of gravitons, the gravitons are not virtual they are real. Graviton particle interactions have very small probability and any virtual diagrams thought up will have a very small probability of acting on other fields. Since in terms of energy the expansion is very small it is only soft real photons that could come from such graviton interactions.

essentially Unruh radiation?

from this link :

The Unruh effect is a surprising prediction of quantum field theory: From the point of view of an accelerating observer or detector, empty space contains a gas of particles at a temperature proportional to the acceleration. Direct experimental confirmation is difficult because the linear acceleration needed to reach a temperature $1~rm K$ is of order $10^{20}~mathrm{m/s^2} .$

Furthermore, the effect is necessary for consistency of the respective descriptions of observed phenomena, such as particle decay, in inertial and in accelerated reference frames; in this sense the Unruh effect does not require any verification beyond that of relativistic free field theory itself. The Unruh theory has had a major influence on our understanding of the proper relationship between mathematical formalism and (potentially) observable physics in the presence of gravitational fields, especially those near black holes.

Let us take an accelerating observer in Galaxy1 expanding from Galaxy2. If this observer measures the vacuum between the galaxies he/she should find Unruh radiation for consistency of general relativity due to the acceleration. Again the accelerating expansion is very small to allow for measurable effects, imo.