recently i saw a patch of dirt on the wall and proceeded to clean it off wif a pc of clean damp cloth. a dark patch was left on the wet area where i cleaned. i thought the patch will be gone after the area dried up but it never did. n the weird thing is that when i stand in front of the wall to look at the patch, it is not obvious but when i viewed it from an angle, from the side, it's very obvious, like a patch of shadow. may i know why is this so? is it water stains? i thought walls can be cleaned wif water? any ways to get rid of the dark patch? n wat's the correct way to clean the wall if i cannot use water? any advice will be greatly appreciated. thanx!