5. The universe is a deadly place. On cosmic timescales – think billions of years – life may be fleeting. All it takes is a single asteroid, supernova, gamma ray burst, or solar flare to render a life-harboring planet lifeless.
6. Space is big. The Milky Way alone is 100,000 light years across, so it’s conceivable that the focused signals of intelligent aliens, which are limited to the speed of light, simply haven’t reached us yet.
7. We haven’t been looking long enough. Eighty years. That’s the amount of time that radio telescopes, which allow us to detect alien signals, have been around. And we’ve been actively searching for aliens for maybe sixty years. That’s not very long at all.