You have to go there in the middle of the game. you will see new areas to explore when you come further in the game or buy starcards from traders!
On the galaxy map, there's a big label pointing towards it once it's available. It becomes available only after completing the mission to Horizon. You can buy starcharts on Illium to access "optional" starsystems in which there may be N7 assignments available (i.e optional minor quests).