There's also the Dark Forest Theory. Any civ advanced enough knows better than to broadcast information that could potentially help other advanced civs spot their location.

Yup, a number of possible solutions. Dark Forest doesn't lead to the likelyhood of us seeing UFOs in Ireland tho.

What about the Dyson dilemma? We've indeed detected "voids", but we're unable to confirm their origin.

If you have a dyson sphere, it glows very brightly in lower wavelengths, such as IR. We don't see that in voids. Additionally, looking back into cosmic history, we can see the formation of voids in the very early universe and their expected expansion.

I find the assumption that any advanced civilization is going to emit external electromagnetic radiation and/or be warm quite curious. Most quantum computing work is at low temperatures, and work on photonics (for example : http://science.sciencemag.org/content/361/6397/57 )is coming along nicely. If we assume an extropian approach of uploading into a more advanced substrate, I find it more likely that they would be quite cold in nature.

ok, fine, then they are blazing in the microwave region. The Dyson sphere will always output the same amount of energy as the original star, just at a lower frequency. Otherwise the temperature would increase continuously.

Pretty difficult to beat the laws of thermodynamics.

QC is a meme.