Fog of war is a common strategy game tactic used to limit the knowledge of a player outside of their immediate realm of influence.
Fog of War is marked by the Astros marked with a Blue grid line and a Red Question mark in the middle, Scouts are able to transmit the Data about these astros to your empire while they are in station in the region. If you don't have a ship/base in the region, you cannot see any astros in that region until you put a ship in this region.
The fog of war prevents the easy scouting of a region, or galaxy. For this reason scouts are often attacked to continue another players blindness to a defenders actions.
Fog of War can be very annoying, but it can be very useful if used properly. If you can always keep a ship in your enemies' regions, and you can keep their ships out of your's, you will have a significant advantage, as they will not be able to see your fleet movements, but you can see their's. It is advised that you always keep AT LEAST 1 scout ships in a region with your enemies in it, so you can monitor their movements and plan accordingly.