A recent article in Technology Review by Nick Bostrom generated a lot of discussion about the Fermi paradox, which states:

I’ll add my 2 cents to this discussion by saying that there’s a possibility that any civilization that becomes advanced enough discovers that physical reality can’t hold a candle to virtual reality and makes the transition (alien transubstantiation, to coin a phrase). This could explain why they haven’t colonized the galaxy, or why we aren’t bathed in their radio communications.

Virtual worlds can be, in theory, both much more pleasant to inhabit, with unlimited freedom and none of the downsides of an existence based on crude physical processes, and also much more energy-efficient. Even without cold computing, it would take a lot less energy for an advanced civilization to do all that it wants to do within a simulation than by moving atoms around.