Intelligence is the question.
According to our own subjective standards.
There might be stuff out there that’s beyond our understanding or comprehension.
No doubt ... but, even if we don't comprehend it ... perhaps, especially if we don't understand it ... we can still, if we ever detect it, judge it to be intelligent if it ...
... transmits signals using technology ... is self aware ... engages in deterministic activities that are not essential to survival.
Liu Cixin, in his book "The Dark Forest" (sequel to "Three Body Problem") has some pretty interesting theories regarding intelligent life in The Galaxy (which are also pretty dark).
1. Life in the galaxy grown exponentially so that The Galaxy may actually be teaming with billions of societies. However, there is limited only be resources in the galaxy, of which there is a fixed amount.
2. There is a vast spectrum of technological societies in The Galaxy. Technological advances come in bursts, they are not linear. So, a technological society that is behind us today may be far ahead of us tomorrow, or vice versa.
3. Any society that broadcasts its location could be a benevolent society or a malevolent rival for the limited resources of The Galaxy.
4. There is no way to determine if a society is benevolent or malevolent without significant communication that would reveal your location to that society. And, even if the detected society IS currently benevolent, there is no guarantee that they wouldn't suddenly become a rival if they find themselves short of resources.
5. So, sufficiently technologically advanced societies would, as a matter of survival of their society, destroy any society that makes its presence known to The Galaxy.
This is a possible explanation for why a galaxy teeming with intelligent life may be quite silent.