As you can see in my signature, I've been working up a theory (based on the real-world Many-Worlds Interpretation) to explain the presence of humans, chickens, and other Out-Of-Universe species that seem to appear In-Universe, as well as other things. The theory states the following:
1. The SW Universe is a parallel universe which has split off from ours, but it did so before/during the Neanderthal times, before the most recent Ice Age.
2. Our timeline still has all the aliens as theirs, but they either haven't discovered Earth yet, they don't have hyperspace tech yet, and/or they live more than 10,000 pc away.
3. In order for 1 and 2 to work together, there must be/have been a sentient being that somehow went back in time to 1,000,000 BBY or earlier and killed the one that invented the hyperdrive, thus creating a third timeline, in which Earth is the sole world of humans.
4. Saturn's moon Mimas resembles the 2 Death Stars, but it doesn't match the dimensions of either (It's bigger than the 1st DS, but smaller than the 2nd). Also, the crater that causes such a resemblance was discovered 3 years after ANH was finished. In other words, if there is an IU Saturn with an IU Mimas, somewhere in the far reaches of the Unknown Regions, and the Yuuzhan Vong passed it before they attacked, then they might not have been surprised when they read about the Death Star in the Galactic history. This one seems to be the least likely, so far.
5. OOU: The Kepler Spacecraft has discovered 1,000+ planets. Some of them are pretty Earthlike, and they may have alien life on them. I also found some Bespin and Yavin Prime candidates. I'm interested in Kepler-186f (Malastare?), KOI-5123.01 (Alderaan?), and 55 Cancri f (Bespin? Yavin Prime?). 55 Cancri f could possibly be an OOU version of Bespin, or possibly Yavin Prime.
6. Out of all the UFO sightings on Earth, there seem to be some legitimate ones (mostly triangular and saucer-shaped). Maybe they've found us, but are hesitant to make true contact.
7. Either Earth's population is completely blind to the force, the force is extremely weak in our area, or there is a hyperspace disturbance that's preventing us from using it. The Local Bubble, a cavity in the Interstellar Medium, may be such a field.
Out of all those, the first 3 are the most likely, and #6 is the most unlikely. We can't prove/disprove #7, because doing so would require sending at least 1 human past the edge of the Local Bubble. That would probably require a so-called "generation ship", similar to a Yuuzhan Vong Worldship, but smaller.