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.
