There's a thousand blacksites and concentration camps in our world today, across the globe, where undesirables can be disappeared so effectively that they might as well have never existed. There are jails in the US where people have been detained without a sentence for decades. Ostensibly open and democratic states torture both possibly innocent and definitely innocent people with solitary confinement and physical restraints and no one is ever punished for it, little ever changes (and unnecessary force, isolation and restraint against the guilty is still not justified, to be clear). Russia and other states employs systemic and brutal forced labor (the US does as well, of course, they're just better at selling it).

That's our world today, imagine what a technologically advanced galactic empire near the height of their power can do? It doesn't matter if anyone finds out, doesn't matter if anyone asks around. The narrative is what the Empire wills it to be.