We do not know what dark matter is, but we do know several things it is not. Among the things that it can not be is terribly strongly interacting with itself. We know this because dark matter appears to surround galaxies with spherical halos, rather than in discs. If it did strongly or even moderately self-interact, it would also be in the disc, because that's precisely why normal matter galaxies are in a disc.
So this excludes, for instance, the idea that there are dark matter civilizations using dark matter machines to send some sort of dark communication that we can't see.
It is not quite impossible to eliminate the possibility that there is some sort of dark radiation we could eventually create and detect. There's currently no particular reason to believe such a thing is possible, but it is not currently excluded necessarily. Whatever forces dark matter interacts with could have its own radiation. But it is difficult to imagine in what way this would be superior to EM-based communication, because, again, EM is just so perfect for communication that it's hard to beat.
It's the question people don't want to deal with because they're too busy being "gee whiz golly gosh" about the possibilities... in what way is this other communication method going to be better than EM? Dark matter communication would quite likely look in practice a lot like neutrino communication does today. It's already possible. We know that because we've built it. We don't call it a transmission method, though, because the bandwidth is soooo atrocious that it's reasonably approximated by "0 bits per second", so we just call it a "detector".
We know one way in which it would be superior - very low interaction rates which ordinary matter which would mean much longer propagation ranges.
And their difficultly is a bonus in itself - if you want to hide your communications as much as possible.
Cost is not always the only metric to optimize. Even today, people try to use quantum entanglement to create communication channels where snooping is detectable.
"very low interaction rates which ordinary matter which would mean much longer propagation ranges."
EM already propagates across the known universe.
"And their difficultly is a bonus in itself - if you want to hide your communications as much as possible"
It is unlikely that such hiding is going to be more effective than existing spread-spectrum techniques, which can already put your communication arbitrarily close to the noise floor, and already exist.
Of course, if you're going to leave science behind you're then going to hypothesize what aliens may have done to somehow break through the noise floor so that's not an option, and, well, this is what I mean by pick one; either use science, or imagine whatever you like, but don't mix the two. There's no reason to believe that the noise floor is penetrable and lots of mathematical reasons to believe it very much isn't.
> EM already propagates across the known universe.
I meant it in the sense that EM will be attenuated by gas clouds and solid matter due to the high interaction rate of EM, while neutrino (or presumably dark matter) much less so.
Since we are allowed to step outside the realm of current known physics I imagined what if we could produce a yet discovered energy that could travel faster then light and could also pass through matter. And lets for the sake of this hypothetical discussion say that we could prove there is no possible way to produce this energy without doing it intentionally, ie intelligent life producing it. I also picture this energy interacting only with itself in a way we can detect(it flashes or something). We could build an emitter and watch for the flash. Since it only interacts with itself and only is produced intentionally and not all over the universe occuring naturally we would know we have detected someone else sending the signal.
So this excludes, for instance, the idea that there are dark matter civilizations using dark matter machines to send some sort of dark communication that we can't see.
It is not quite impossible to eliminate the possibility that there is some sort of dark radiation we could eventually create and detect. There's currently no particular reason to believe such a thing is possible, but it is not currently excluded necessarily. Whatever forces dark matter interacts with could have its own radiation. But it is difficult to imagine in what way this would be superior to EM-based communication, because, again, EM is just so perfect for communication that it's hard to beat.
It's the question people don't want to deal with because they're too busy being "gee whiz golly gosh" about the possibilities... in what way is this other communication method going to be better than EM? Dark matter communication would quite likely look in practice a lot like neutrino communication does today. It's already possible. We know that because we've built it. We don't call it a transmission method, though, because the bandwidth is soooo atrocious that it's reasonably approximated by "0 bits per second", so we just call it a "detector".