It has a belief that capturing the resources of the galaxy would not increase its security nor further its objectives. It doesn't mind interfering with us, which it is implicitly doing by hiding its presence and giving us the Fermi paradox which in turn influences our beliefs about the Great Filter.

We should also consider what beliefs or knowledge it could have that would cause it to stay home. For instance:

• The Universe is a simulation and making parts of it more complex or expensive to simulate would shorten the life of other complex parts, would crash the simulation, would fail because the simulation wouldn't allow it, would draw undesired attention from the simulators, etc.
• If their civilization spread to other stars, lightspeed limits would make them effectively independent, and eventually value drift or selfishness would cause conflict or would harm their existing goals
• There is some danger out there, perhaps a relic of an even older civilization, and the best course is to hide from it; humans have not yet been attacked because we only started beaming out radio signals less than a hundred years ago

It may have discovered some property of physics which enabled it to expand more efficiently across alternate universes, rather than across space in any given universe. Thus it would be unlikely to colonize much of any universe (specifically, ours).

Similar to some of the other ideas, but here are my framings:

1. Virtually all of the space in the universe have been taken over by superintelligences. We find ourselves observing the universe from one of these rare areas because it would be impossible for us to exist in one of the colonized areas. Thus, it shouldn't be too surprising that our little area of non-colonization is just now popping out a new superintelligence. The most likely outcome for an intelligent species is to watch the area around them become colonized while they cannot develop fast enough to catch up.

2. A dyson-sphere level intelligence knows basically everything. There is a limit to knowledge and power that can be approached. Once a species has achieved a certain level of power it simply doesn't need to continue expanding in order to guarantee its safety and the fulfillment of its values. Continued expansion has diminishing returns and it has other values or goals that counterbalance any tiny desire to continue expanding.

1. Alien AI is using SETI-attack strategy, but to convince us to bite and also to be sure that we have very strong computers which are able to run its code, it makes its signal very subtle and complex, so it is not easy to find. We didn't find it yet but will soon find. I wrote about SETI attack here: http://lesswrong.com/lw/gzv/risks_of_downloading_alien_ai_via_seti_search/

2. Alien AI exist in form of alien nanobots everywhere (including my room and body), but they do not interact with us and try to hide from microscopy.

3. They are berserk and will be triggered to kill us if we reach unknown threshold, most likely creation of AI or nanotech.

2 may include 3.

The superintelligence could have been written to value-load based on its calculations about an alien (to its creators) superintelligence (what Bostrom refers to as the "Hail Mary" approach). This could cause it to value the natural development of alien biology enough to actively hide its activities from us.

May be the AI had existed in the Galaxy and halted by some internal reasons, but leaved after it some self-replicating remnants, which are only partly intelligent and so unable to fall in the same trap. Their behavior would look absurd to us, and that is why we can't find them.

given our current observations (I.e. that there is no evidence of it`s existence)?

Our current observation is that we haven't detected and identified any evidence of their existence.

Another option: Maybe they're not hiding, their just doing their thing, and don't leak energy in a way that is obvious to us.

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).