I’m not saying it’s brilliant, I’m just saying that the galaxy is pretty friggin’ big and exploring a big chunk of mostly empty space might not be a big priority once you’ve already mapped most of the other half of the galaxy. I’m guessing that the mapping of the “eastern” part of the galaxy was achieved through the races that the people from the core-world met along the way. F.ex. you find one planet that’s inhabited, and you learn about 10 more planets from them. Exploring those 10 you discover 5 more, and from one of their civilizations you discover 10 more, etc, etc. And before you know it you established trade routes, political ties, and explored a large part of the galaxy. The “western” half on the other hand may never have had that first meeting which eventually lead to what is an almost exponential growth in knowledge/expansion. Without that first meeting you are essentially just faced with what is 99,99% empty space and no real starting point for exploration. So even with lightspeed technology it’s far from impossible to spend the next 4000 years not knowing what lies in the “western”-half of the galaxy even if it is right next to you.
(And as for the whole Viking-analogy goes. During a roughly 3-4000 year period of seafaring capabilities there was only one known expedition over the Atlantic ocean. They eventually figured it wasn’t worth their time since everything they needed as far as trade went was east of them anyway, so they simply decided to leave America alone, and for another 1000-ish years it was assumed to have just been some small-ish, uninteresting island somewhere west of Greenland.
Also space is a hell-of-a-lot larger than any areas we’ve ever had to explore on our own planet. At least we can make a full journey around the planet if we get lost. If you explore space however, you could be travelling through nothingness for an eternity.)
Sorry but your analogy is not very convincing. It is completely out of scale. Vikings might have ventured briefly to America but that doesn’t mean they explored/settled any significant portion of it, let alone half of the world (which is required to meet our Star Wars example). They effectively explored and used a very small portion of the world. On the other hand, our Star Wars example dictates that half of the galaxy was completely explored (and used) while the other half was unknown. If you have the capability to discover/colonise half of the world then the other half will naturally follow very soon, as in the case of 16th-17th century European discoveries and colonisation.