To clarify a bit more.
Those brighter areas you notice are what is often called "LoS" or Line of Sight, that means that anything that is within those areas is readily seen without having to move an army or scout into that area.
Factors that affect LoS are;
1. City sizes, the larger the city is the more you gain in LoS (small amount overall)
2. Guard Towers (aka GT's) also improve LoS (largest amount overall)
Max LoS gained from GT's is 2,000 (2K), you can add more but you will not gain more LoS.
3. Being in a Kingdom. Every city and every army/scout whether they be yours or a KD (Kingdom) member gives you LoS. So if your KD is well established on another map before you get there, when you get onto that map you will be able to see everything they can.
The darker areas represent areas that the LoS has not penetrated, in other words you need to move into that area with an army or scout, or build a city near it and drop (build) GT's to gain sight into that area.
Any city that has its gates closed is visible on the entire map whether or not they are in your LoS, there are also other factors that may be visible outside your LoS, but you will learn those as you play this great game.
Hope this helps.