Since some people have opined that maybe we're not alone in the universe, I'll write down the strongest argument in that I can think of in favor of this position. (To win, you must fight not only the creature you encounter; you must fight the most horrible thing that can be constructed from its corpse. )
The strongest reason that aliens might be invisible to us is that they are deliberately hiding. In fact I think that this is the only plausible reason.
Why would they be hiding? Well, they might be frightened that they're in a simulation, and that the simulators have some crude algorithms that search massive tracts of space (many, many hubble volumes in size) by simply looking for inhomogeneity on the galaxy or supercluster level. The advanced aliens don't want to get caught by the simulator.
This explanation would still work even if we're not in a simulation: the threat of it is enough. Even a small probability that we're in a simulation given the intelligence and data that an advanced alien civilization would have might be enough to offset the (perhaps small?) advantages of expansion, especially given recent work on bounded utility functions.
EDIT: This could be combined with the singularity hypothesis: perhaps all superintelligences, convergently decide not to expand.
While I'm not in any way an expert in simulation making, wouldn't it seem just a bit too convenient that, in all the monstrous computing power behind making the universe run, the Overlords couldn't devise a pretty clever and powerful algorithm that would have found us already? Maybe you can help me see why there would only be a crude algorithm that superintelligences should fear being caught by, and why they wouldn't have considered themselves caught already.
Apart from this, I'm in agreement with other commenters that a stronger argument is the vastness of space.
An uplifting message as we enter the new year, quoted from Edge.org:
A few thoughts: when considering the heavy skepticism that the singularity hypothesis receives, it is important to remember that there is a much weaker hypothesis, highlighted here by Tegmark, that still has extremely counter-intuitive implications about our place in spacetime; one might call it the bottleneck hypothesis - the hypothesis that 21st century humanity occupies a pivotal place in the evolution of the universe, simply because we may well be a part of the small space/time window during which it is decided whether earth-originating life will colonize the universe or not.
The bottleneck hypothesis is weaker than the singularity hypothesis - we can be at the bottleneck even if smarter-than-human AI is impossible or extremely impractical, but if smarter-than-human AI is possible and reasonably practical, then we are surely at the bottleneck of the universe. The bottleneck hypothesis is based upon less controversial science than the singularity hypothesis, and is robust to different assumptions about what is feasible in an engineering sense (AI/no AI, ems/no ems, nuclear rockets/generation ships/cryonics advances, etc) so might be accepted by a larger number of people.
Related is Hanson's "Dream Time" idea.