So says Stephen Hawking:
One option is that there likely isn’t life elsewhere. Or maybe there is intelligent life elsewhere, but when it gets smart enough to send signals into space, it also is smart enough to make destructive nuclear weapons.
Hawking said he prefers the third option:
“Primitive life is very common and intelligent life is fairly rare,” he then quickly added: “Some would say it has yet to occur on earth.”
There is no real evidence of life existing elsewhere (save for maybe the controversial ALH84001 Martian meteorite, but given the vastness of the universe and how little of it we have explored, our data is so limited that we can only speculate. It is widely believed that liquid water exists on Jupiter’s moon Europa and Saturn’s moon Enceladus so life would be theoretically possible on either. But until we can do more exploration of space, we can only speculate. Until then, I’m inclined to agree with Hawking: we probably are not alone.