Besides, if you think the aliens can all agree to stop expanding to hide from theoretical simulators then I don't see why some other meme couldn't do the trick too.
I can't think of anything else that would affect every single alien civ out of trillions; in fact, even this explanation seems unlikely to account for no expansion from every single one of trillions of civs.
I think wireheading,
Perhaps we should regard the fermi problem as evidence in favor of wireheading?
religious seclusion
I think that the "religious seclusion" idea is not a good bet. Remember, it has to work with really rather high fidelity.
or some other memetic effect could keep them from expanding
Some other mimetic effect seems more promising.
If the simulators are looking for intelligent life, then they may be removing it when they find it, to allow the rest of the universe to continue on producing other original life forms. Then, there would be intelligent life in hiding, and other intelligent life would be selected against.
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.