Define "man"
Define "aliens."
Let's just pretend for the sake of keks that we are in the 40k universe and are some lost offshoot of humanity for some reason. Wouldn't, upon rediscovery by the imperium, those who suppressed the truth, knowingly, be guilty of heresy against the god emperor?
For that matter, how would reintegration into the imperium be done in such a way as to actually benefit the imperium?
Perhaps complicating all of this is that the "humans" of the imperium were not genetically modified to survive on a planet circling a generally hotter star.
Unless you intend to go the firmament route, in which case, you're blatantly excluding the angelic angel and the idea the government would be hiding the proof of God being real (or something along those lines).