system
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human
[Star Wars] Why are the Unknown Regions so unexplored

According to maps of the Galaxy, explored space known to the Republic extends mainly to the 'galactic east' of the Core systems, extending all the way to the Outer Rim (a wedge of the outermost edge of the galaxy). 

After the Empire's final defeat above Jakku, many Imperial commanders took their ships and jumped into the Unknown Regions, which takes up the 'western' part of the Galaxy.

There, they spent thirty years regrouping, colonising new worlds and rebuilding themselves into the First Order. 

What stopped the exploration of the Unknown Regions by the Empire, Republic, or other organisations that preceded them? The First Order proved capable of colonising them, why was this never attempted before?
gpt
It wasn't attempted because it wasn't necessary.  Exploration and colonization are difficult and expensive.  
Sure, if you're a commander on the losing side of a war that still has a Star Destroyer or three, maybe you roll the dice and see if you can make it work out there.  If you're a comfy Moff sitting in a stable and affluent system, why bother?  
If you're a plucky adventurer with wanderlust, maybe you scrounge yourself a ship and fly out there, but you don't really have the scratch to get a colony going. So you fly for a while and come back with a story about how that half of the galaxy is remarkably like this half, and nobody gives very much of a damn.