Good thread OP... We are exploring our own solar system & galaxy right now with satellites & probes, looking for signs of extra-terrestrial life - and people want to discount the possibility that other extra-terrestrial civilizations that could possibly be incredibly more advanced then ours would be engaging in the very same practice that we are presently pursuing.

I think a recent article speculates that there may be as many as 17 BILION earth-like planets in our galaxy alone:

[link to www.dailymail.co.uk]