The amount of dark energy in the universe is pinned down fairly precisely by fluctuations in the cosmic microwave background, if I understand correctly. So in that sense there's very good evidence of its existence.
What I'm wondering is, could there ever be a laboratory-sized experiment that demonstrates its effects? For comparison, Pound and Rebka demonstrated the existence of gravitational redshift over the height of a five-storey building. What would be the smallest equivalent for showing the existence of dark energy?
For instance, suppose I have two super-light ping pong balls floating in space. (On the ISS, say, or on board Voyager 1.) Would dark energy make them drift apart?