[allendale]

If there are billions of galaxies, and the distance between most of them are hundreds of millions of light years then two things are fairly certain. 1)There no doubt are many civilizations out there.2) Given the laws of physics and the fact that the speed of light is the universal speed limit, there is no way any biological entity or mechanical construction from an alien civilization has ever or ever will visit earth. However Earth has been transmitting data in analog form for over one hundred years and in algorithmic digital form for over thirty. It is quite possible that alien civilizations have linked up with one another by relaying data (with long travel times) to each other. Perhaps in thirty years or so some computers on earth will be “infected” with a truly wondrous virus.

[Svartalfiar]

However Earth has been transmitting data in analog form for over one hundred years and in algorithmic digital form for over thirty. It is quite possible that alien civilizations have linked up with one another by relaying data (with long travel times) to each other.

There's almost no way any alien species has even heard our radio signals yet. We've been broadcasting around 100 years, which gives us ~200 light-year bubble where those radio waves have reached. (Slightly different from us moving while sending, but not majorly so.) Also, those broadcasts were not aimed into space, but just random scatter, so they'd be crazy weak by now, if not completely dispersed. We haven't really been actively transmitting into space until about 40 years ago (Arecibo). So our bubble is even smaller. We've only been listening for about the same time, which gives us a very narrow band of distance combined with time in which a transmission would reach us. (Too close/long time is past us, too far haven't reached yet.)

Also, would a highly advanced species still be transmitting/listening on the same frequecies we use? I would hazard a guess that technology would advance more into gamma or similar communications, as the higher frequency gives you more data bandwidth, so more effective communication. And higher advancements might use something entirely different. An ansible perhaps.

Unless some form of faster-than-light travel has been developed (I would assume that's more difficult than FTL communication), it would take millions of years for an alien species to reach the Earth, much less find us out of all the other stars/planets nearby. While they are moving at FTL speeds. And then we're also assuming they developed, adapted, and technologically evolved enough to get to that point. As rare as microbial life is, general animals is even rarer, and intelligent life rarer still, and intelligent life that advances to the stars and beyond even rarer. Remember, all this also had to happen a long time ago for them to even be at the point where they may be arriving in our arm of the Milky Way. Maybe they already came by and left a million years ago, passed by.

And I'm also forgetting, we need an oxygen-carbon based lifeform. If it's some kind of methane, or sulpheric, or other lifeform, they'll be looking for something more like Venus or Neptune. If they come from a much higher/lower gravity. More extreme (for us) cold or heat. Their habitable zone may not include where we are relative to our sun.