in magazines and on TV they sometimes say that the universe is expanding. But how can the universe expand if it is already infinite?

Answer

Twice infinity is also infinity!!

The expansion means that the distance between two galaxies expands with time. An amount of mass that covers a certain volume at one time later covers a volume that is larger. And that is the case everywhere: the whole universe is therefore getting bigger. And so even if it initially enriches infinitely.

Just do it with (natural) numbers. If you multiply all the numbers by two, they all got bigger. And yet they were all already in the initial set of natural numbers…