The Unknown Regions, also known as Gyaros to the Chiss Ascendancy or Ancile Regnum to the Sith, or Katperial Space to the Katpire and the Catpire, was a massive region in the "north" of the the galaxy, that had been left largely unexplored by all the galaxy's major powers up until the late 60's ABY.
Despite its seclusion from the greater galaxy, the Unknown Regions was home to several major galactic powers. The most well known of these was the Chiss Ascendancy, which maintained a minimal relationship with each of the major galactic powers, but refrained from becoming fully involved with them, preferring to keep to their own affairs. Both the Galactic Empire and New Republic had allocated great amounts of funding for the exploration, but little information could be afforded from the blackness of the Unknown Regions.
This gap in the archives of the Galactic Senate meant that several powers could continue to grow and strengthen without the inevitable interference that negotiations with them would cause. One such power was the Sith Order of Decreto.
The Sith Order of Decreto, although originally from a part of the galaxy known the the Galactic Republic et al, had made there way into the Unknown Regions in the year 5,000 BBY and remained their for many thousands of years. They quickly hid themselves away from even those who were their neighbors, becoming so deeply secretive (but influential) that they could direct the affairs of the galaxy without anybody knowing anything about them.
After many years of seclusion (not including the occasional encounters) the Sith Order of Decreto under the direction of their Emperor Darth Abeonis (a former Jedi) revealed themselves to the galaxy in the form of an invasion of the Chiss Ascendancy. In the years that followed, the Unknown Regions were opened up much more than they had been previously; with travel within the Sith Order of Decreto relatively easy, the black market expanded into the Unknown Regions. It was this market that allowed for the Jedi Cerras Ki to survive during his seven year journey, and to procure his weapons.
Long after the end of the Unity era and deep into the Dark Age the Unknown Regions were eventually opened up, to a certain degree, and become largely known to the peoples of the galaxy, although great swathes remained hidden away on the orders of the Sith Council. By the mid Dark Ages, almost the entire region had been explored, but, as the last part of the galaxy that was explored, the various map-makers decided to keep the name Unknown Regions, for symbolic reasons.