I am just reading the review article Advances in exoplanet science from Kepler (arxiv preprint: http://arxiv.org/abs/1409.1595), and I found a remarkable paragraph (last paragraph in section "Properties of planetary systems", page 341):
Just as important as the discoveries made by Kepler are its non-discoveries. So far, Kepler has found no co-orbital planets, which share the same average semi-major axis — like the Trojan asteroids found accompanying Jupiter and the Saturnian satellites Janus and Epimetheus. It has also found neither exomoons nor 'binary' planets orbiting one another.
There is no further explanation or interpretation on that.
What does the authors want to say with this remark? Do they infere that due to the non-discovery, the probability of such objects is very low?
I'm interested especially why the authors think this is important? What does it say?
Edit (October 2018):
I would like to update this question, to mention that that now - four years later - an exo-moon has very likely been discovered by Alex Teachey and David Kipping, using Kepler data (with subsequent observations with Hubble).