My Acropolis can store 15000 but when I got raided my resources were around 7000. I thought they were safe and hidden and that what the game says, so I don't understand why should I lose resources?

When you raid a City that has all of its resources hidden, you will get a small percentage of those Resources regardless - however, they are compensated by the game itself and are not taken out of the City's coffers. At the same time the player who is being raided will see results on the battle report regardless, so what you are seeing is the reward that your attacker received, but it was not taken out of your own Resources.

Thank you for the clarification. This makes perfect sense to me now!

Glad to hear that!