It doesn't has to be one civilization. It just needs to be a single species or multiple different species from other planets that have colonized a large fraction of the galaxy. Even with sublight propulsion a single species would be able to colonize an entire galaxy within 10 million years. The Milky Way itself is about 13.2 Billion years old. It makes sense that any species that wishes to increase its likelihood for survival would colonize as many planets in as many different systems as possible. The political structure governing these worlds has nothing to do with it. Imagine if you had multiple species per galaxy, then the galaxy could be colonized that much faster. Now 10 million years is a very small fraction of 13 billion so we would expect a pretty high likelihood that in a random sample of 100,000 galaxies that are each about 13 billion years old we would see at least one galaxy giving off extra IR radiation because of all the extra energy being used and/or produced by a sophisticated multi system species. Also it doesn't need to be the entire galaxy. Even a large fraction like a quarter or more should still give off a detectable amount of extra black body radiation. What this tells is that either there is so little intelligent life out there that the probably of ever making contact is basically zero or that there is some sort of Great Filter that prevents intelligent species from ever reaching that stage of development. I find this horrifying because either the universe is a baron lonely wasteland or that there is something very fundamental to the universe that we don't know about and it's just a matter of time before it kills off our entire species. Of course the more optimistic view would be that the fundamental laws of thermodynamics don't necessarily apply to the technology of such a highly developed civilization, but this seems so unlikely that it really wasn't worth me mentioning. I just thought it would be nice to offer some hope.