system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the universe is expanding, aren't new stars being born all the time? And I guess they would be even if it wasn't, so why is it said that the universe will be completely dead one day?
gpt
The idea is that, while the universe is expanding in size, the amount of matter and energy in it is finite. That whole, can't be created or destroyed thing. 

What happens when you have a set amount of something in a container that's getting bigger? 

Eventually the universe will be so massive and stretched out that there'll be mere ones of atoms per cubic kilometer. 

Not much of a universe at that point, is it? Certainly not one full of stars and planets and junk.