>>7
I admit I am completely ignorant as far as this subject goes, but I believe the idea is that while the universise is infinite, there is infinite void but the amount of matter is a more tangible number. So within the universe, you could construct a ball of finite radius which contains all matter. If you were to optimize this ball to minimize radius while still containing all matter, the ball itself will be expanding while the universe containing it is still infinite.
Waiting to be corrected...