It’s not really possible to say that it is certain given we haven’t looked very hard or for very long. We don’t even know if there’s life in nearby places like Enceladus.
It’s not really possible to say that it is certain given we haven’t looked very hard or for very long. We don’t even know if there’s life in nearby places like Enceladus.
We don't need to look hard. If interstellar travel is possible and someone in our Galaxy have developed it in the past, the whole Solar System including Earth should be already colonized.
There are many other answers to why it isn’t outside of “they should have done this but haven’t and therefore cannot exist”.