kamalayka said:
Who is to say that the Emperor actually died at the end of Episode VI?
He didn't. He transferred his essence into one of the many clone bodies he had grown for him on the planet Byss. He only died once and for all after he ran out of clone bodies and the Jedi Empatojayos Brand sacrificed his life to forcibly carry Palpatine's soul into the netherworld of the Force.
why would the Empire, with all of the massive ships and vast legions of troops, suddenly decide to call it quits?
It didn't. The uprisings shown in the SEs were swiftly quashed, the insurgents rounded up and shot by stormtroopers. The Empire stayed pretty strong following the Battle of Endor, really, only losing its power as it lost more-and-more territory to the nascent New Republic in the years following.
Out of the entire galactic population, were Vader and the Emperor really the only two on the dark side of the Force?
Nope. There were countless Dark Jedi spread throughout the galaxy, most in service to the Emperor. There were also non-Jedi darksiders, like the Nightsisters on the planet Dathomir, running around as well.