(Just to be clear, I am a fanatic space buff. Just discouraged by current events and things I've learned over the years.)

The primary sign that SETI has been looking for is radio/tv/other Electromagnetic (EM) radiation from other civilizations at the same approximate development as ours.

But a year or so ago there was an article published that found that our own EM radiation fades into the cosmic background within about two lightyears. Beyond that it's completely undetectable against the emissions of everything "behind" us.

So there could be a thriving civilization at the same level as us in the Centauri system, 4 lightyears away, and they'd have no idea we were here.


Also, the scale of space is mind boggling. If the Sun were a ball 3 feet across, then the Centauri system would be 25,000,000 miles away. (On that scale, Earth is about a third of an inch.) Our own galaxy is approximately 100,000ly across, or, on this scale, about 7,000,000,000,000 miles.

While stars in the core are close together, often less than a lightyear apart, the radiation levels there would prevent advanced life from developing. Probably even any life at all.

Out on the rim, where we are, 5-10ly apart is normal for stars that aren't in a cluster. Stars in clusters like the Pleides or the Orion Nebula can be fractions of a lightyear apart.

Our galaxy is big. Very, very big. Even our nearest intelligent neighbor (if there are any) is most likely hundreds of lightyears away.