"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.

If it doesn't interact with gas clouds how do you build an efficient detector to receive it?

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.

But then you have the issue that the communication doesn't actually interact with the detector you've built to receive signals.