The most obvious and least likely possibility is that the superintelligence hasn't had enough time to colonize the galaxy (i.e. it was created very recently).
That's very unlikely. The universe is billions of years old, yet it would take only mere thousands of years to colonize the galaxy. Maybe millions if they aren't optimally efficient, but still a short time in the history of Earth.
For a moment lets assume there is some alien intelligent life on our galaxy which is older than us and that it have succeeded in creating super-intelligent self-modifying AI.
Then what set of values and/or goals it is plausible for it to have, given our current observations (I.e. that there is no evidence of it`s existence)?
Some examples:
It values non-interference with nature (some kind of hippie AI)
It values camouflage/stealth for it own defense/security purposes.
It just cares about exterminating their creators and nothing else.
Other thoughts?