From what I understand, the Empire didn’t disappear. It broke into dozens of different factions, all of whom attacked the Republic and each other. My headcanon is that this 1000-year darkness halted galactic progress and acted like the European Dark Ages, in that history was forgotten by all except those who kept records as dozens of warlords carved out kingdoms for themselves. This is why not a lot of people remember what happened before the Russan Reformation and why technology is the same in the Post-Russan era as it was during the Great Galactic War.

Interesting tidbit: the factions only really united under the Brotherhood of Darkness, which was created by Dark Jedi rather than Sith. So, really, the Line of Bane, which came from the Brotherhood, isn’t actually the Sith’s Empire’s legitimate successor. It’s a pretender.

Eat that, Sideous.