Light can't exist without darkness, and the other way around. As long as there's darkness, there are Heartless.

Kingdom Hearts is the heart of all worlds. The heart of everything, really. When you destroy a Heartless, the heart that it had stolen returns to Kingdom Hearts.

[ She's not going to mention that you need a Keyblade to do it. She doesn't trust him enough for that yet. ]

So think about it: Thousands upon thousands of Heartless have been killed over the years-- and all those hearts are now in Kingdom Hearts. The heart of everything, super powered by all the hearts that it has obtained.