As with all games some kind of story must be established, so that everyone knows the rules of the universe.
In this case, being a "space simulation" type game, it'll have to be explained, as well as restrict the player so they can still have fun without trying to play it as if it were a star wars game.
So as to why the planets in the system aren't accessible (if that's how it's left) we can say that the space outside of the Asteroid belt is patrolled by Star Destroyer sized ships, belonging to the "Alliance", "Empire", "Coalition" or whatever the huge civilized race is, and venturing too far outside the belt is suicide...or perhaps there is just no reason to go there.
The time span the game takes place must be a viably far distance into the future, if it is a future Earth, although it might be easier to just say "in a galaxy far far away" (though it doesn't really matter if it was a long time ago in that case)
The biggest 'stretch' of known science is the concept of "I-morts".