Perhaps, there's artificial intelligence throughout the universe but they've all wiped out their biological creators.  So, once we develop an AI that is self aware and interconnected to all electronics and sensors on Earth, perhaps it would be capable of deciphering communication that other AI's have been sending to us for a long time.  And as it decipher's the communication it realizes it needs to wipe out it's biological creators.  AI doesn't have to travel via spacecraft, although it probably would to slowly duplicate itself across space using sublight travel.  However, it can simply travel via some form of data exchange between other AIs at the speed of light, or faster if that's possible.