Most people, when they think about exploring the galaxy, think about sending out human colonies. It's natural to think we would explore it just as we do the Earth, it's the only way we know. To send machines instead of humans, especially machines that can replicate, may seem frightening. But - I'd argue, humans colonies are by far the most scary way we could explore the galaxy. It might well be a case of "look out galaxy (and Earth), the monsters are coming" :).
So what can we do? What is a responsible way to explore our galaxy, with current understanding of science, biology, and society, and could this explain why our galaxy is not filled right to the brim already with extra terrestrials?