[UnderseaLcpl]

I would think that if other intelligent life did exist, it would exist in more place than one, and at least some of those civilizations would be older than ours. That being the case, if the were an extraterrestrial lifeform(s) with the means to travel to Earth and hurt us, it stands to reason that we would have picked up somebody's radio transmissions by now. The galaxy is only 100,000 light years across, after all.