If there is life on other planets, why haven't we been visited by aliens?
I wonder this sometimes. This image explains in a powerful way why that might be; click on it and pause for a minute to reflect on what a tiny portion of our own galaxy would have been reached by radio waves broadcast from the earth. Our first radio broadcasts were made a century ago, and so could have traveled a hundred light years. The Milky Way is more than 100,000 light years across.