It depends on how long the alien civilization is allowed to last. If it poofed into existence 1b years after big bang and then spread at 0.6c for 7b years (leaving ~7 more billion years for their light to reach us), then they might occupy a big enough fraction of the sky, that it wouldn’t be entirely obscured by the milky way or any other single galaxy (not that I checked the math). But that’s very generous.
Otherwise, we may as well consider them stationary. In that case, if their light really couldn’t pass through the denser parts of galaxies, they could use some other signal, like neutrinos or gravitational waves. Not sure how to make that many neutrinos. For the latter, I suspect making two massive black holes and making them merge might not be that hard.
I imagine you could even do it without moving solar-mass objects—just build two small-ish black holes and launch them on precise trajectories such that they would eventually collide, while eating up many smaller objects and gaining mass along the way. This assumes that the aliens have perfect information about their own galaxy. Each trajectory would of course take a very long time, but if you launch many, you could produce a repeating signal. Unless they run out of stars to use.
It depends on how long the alien civilization is allowed to last. If it poofed into existence 1b years after big bang and then spread at 0.6c for 7b years (leaving ~7 more billion years for their light to reach us), then they might occupy a big enough fraction of the sky, that it wouldn’t be entirely obscured by the milky way or any other single galaxy (not that I checked the math). But that’s very generous.
Otherwise, we may as well consider them stationary. In that case, if their light really couldn’t pass through the denser parts of galaxies, they could use some other signal, like neutrinos or gravitational waves. Not sure how to make that many neutrinos. For the latter, I suspect making two massive black holes and making them merge might not be that hard.
I imagine you could even do it without moving solar-mass objects—just build two small-ish black holes and launch them on precise trajectories such that they would eventually collide, while eating up many smaller objects and gaining mass along the way. This assumes that the aliens have perfect information about their own galaxy. Each trajectory would of course take a very long time, but if you launch many, you could produce a repeating signal. Unless they run out of stars to use.