The canon novel Tarkin mentions that the Sith were driven into hiding before the creation of the Republic:
Cynosure of the Greater Seswenna sector of the Outer Rim, Eriadu could trace its history to the earliest era of the Republic. At that time, the galaxy’s dark age had ended, the Sith had been defeated and driven into hiding, and a true republic had emerged from the ashes. With a member of House Valorum presiding as Supreme Chancellor, a pan-galactic Senate had been created, and the military had been disbanded.
The Sith once ruled an empire and fought an open war against the Jedi: the Jedi-Sith War. It was after their defeat by the Jedi that Darth Bane reformed the Sith and went into hiding.
Legends has a much more detailed backstory for the Sith. The Sith had multiple empires. The last Sith Empire before Darth Sidious' forced the Galactic Republic to sign the Treaty of Coruscant, which divided control of the galaxy between the Republic and Sith Empire:
Canon has largely borrowed elements of the Legends stories.