I'll say up front, I really appreciate your doing a forum search first. It'd save a lot of people a lot of repetitive answers
The Sith Empire has canonically been expanding and fading throughout history for thousands of years, though Palpatine certainly implemented a distinctive and strong one. Some history is laid out on Wookiepedia
. The Galactic Empire
was Palpatine/Darth Sidious's creation; the Empire of SWTOR's timeline
, created by Darth Vitiate (SWTOR's Emperor) came much earlier.