This became a problem with the start of the NJO series, where you couldn't pass though the border of the galaxy unless you were in one particular location. After that, they piled on more reasons why a large section of the galaxy was almost unknown. Now that the NJO is non-canon, this isn't a problem any more. They just make the Outer Rim/Wild Space filled with unknown systems. I think Rise of the Rebels even says you can travel the space lanes in the Outer Rim for years without meeting anyone. They're making the OR out to be the big unknown in canon, apparently. Vast emptiness in-between known systems. That way you could have characters traverse the galactic disk and still run into new things.