"They exist but we don't have the tech to detect them"?
That one shows up in fiction every now and then, but If they're galaxy-spanning, there's no particular reason for them to have avoided eating all the stars unless we're completely wrong about the laws of physics. The motivation might not exactly be "hiding," but it'd have to be something along the lines of a nature preserve; and would require a strong singleton.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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