"Until then" means up to that time, either in the future or in the past. It marks a limit and tells what remains true or what to do before that moment. It also works as a polite sign-off, like saying goodbye for now.
"28 years ago, Penny Marshall told us a story about women playing professional baseball that up until then had been largely overlooked. Source: Internet
After further questions from journalists, he confirmed that the regulations included the border crossings through the Wall into West Berlin, which he had not mentioned until then. Source: Internet
A Lahore flight on 5 August provided the first photographs of the Baikonur Cosmodrome near Tyuratam : the CIA had been unaware of its existence until then. Source: Internet
And Algeria until then was the main source of oil and gas destined for the French market. Source: Internet
But until then, just scroll past anything that sounds too good to be true. Source: Internet
At that time, demonstrations of the method's efficiency drew huge public interest, because the effect on patients, especially children, who until then were bound to die, seemed almost miraculous. Source: Internet