This is really a simple mystery. Is there other intelligent life out there in the universe? Carl Sagan reminds us that if we exist, then, no matter how rare intelligence is in the universe, given how huge the universe is, we must have many neighbors out there somewhere.
Frank Drake, an astrophysicist, created an equation that helps figure out how much intelligent life there is in the universe, and estimated that if only one in a billion planets has intelligent life, then there must still be over 6 billion planets with intelligence on them.
Enrico Fermi, however, pointed out that if life is that common, then it is virtually impossible that we haven’t yet detected any signs of other intelligent life in the universe. So, the real mystery is this: what is it about Earth that makes no one want to play with us?