The first extensive search for galactic-scale alien civilizations, looking for their expected waste heat signature in the middle infrared. Across 100,000 nearby galaxies, none were found.
"Our motivation is to use this catalog to perform the first extragalactic search for waste heat from galaxy-spanning alien supercivilizations. ... We find that there are no galaxies in our sample of 1x10^5 galaxies with fit values of γ > 0.85, meaning that no galaxies resolved by WISE contain galaxy-spanning supercivilizations with energy supplies greater than 85% of the starlight in the galaxy (unless this energy is not primarily expelled as light in the WISE bandpasses)."
My own hypothesis, which I haven't found anywhere else, is that once a civilization begins to settle surrounding stars, it does so at near light speed, so that there is only a tiny interval of time between evidence of their existence reaching some star, and their colony ships doing so, in the process precluding the possibility of another civilization emerging there (note: this assumes no malevolence, only the exploitation of unclaimed resources). Hence, the birth of a civilization is overwhelmingly likely to occur prior to this evidence arriving, and when it wakes up and looks at the sky for the first time (as we are now), it will see nothing, even if there are other civilizations out there, far away.
A corollary is that births of civilization are spatially sparse and occur nearly simultaneously throughout the universe, probably when the conditions begin to allow it (say, after several star generations have produced appropriate heavy element concentrations, and when gamma ray bursts become infrequent enough to allow the evolution of complex life to progress uninterrupted).
But if this hypothesis is wrong, and civilizations expand at a rate more like 1-10% of the speed of light, then that is still fast enough such that other galaxies should be fully colonized by now, yet not so fast that our own civilization couldn't exist, and see them. Hence, this study may have implications for the long-term survival prospects of any civilizations (including our own), and/or for the prevalence of complex life in the universe, per Great Filter theory: http://mason.gmu.edu/~rhanson/greatfilter.html