Was it ever explained how Sidious was able to peer into TWBW in the first place? I'm guessing he already knew of its existence prior to the episode of Rebels, but just couldn't properly access it right?
Like, how was he able to conjure the initial portal and see Ahsoka and Ezra? Why were his powers able to enter that realm, but Sidious himself could not?
Oh, Palpatine undoubtedly knew about TWBW well before the events of Rebels -- he just didn't know how to physically access it. That's why he needed Ezra to act as his 'tether' to the realm in order to enter it in the flesh.
As for how Palpatine was able to peer into TWBW: I assume it was similar to how Yoda was able to when he and Ezra conversed in season 2(Filoni stated that all took place in TWBW as well.) Except Palpatine clearly used Sith sorcery to do so.
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."