In Endless Space every system has its own border line, which is constantly growing due to its development. Also fleets creates a space around them, which is a radar to detect enemies.
On the screen above you can see those elements.
The darker circle around the fleet (1) shows the range of revealing alien units. It can be enlarged by the technologies (Science branch) and usage of the support modules.
Example of the technology:
The red space around the system (2) is a border line of the system and all the empire: it growths according to the development of the planets.