Infinity theory

When I read, I noticed a contradiction between my knowledge and book knowledge. It turns out: If after a supernova explosion there are elements for the birth of a new star, the question arises: If after a supernova explosion there are elements for the birth of a new star, then why does the universe not exist forever? So, suppose: If the universe exists forever, then a contradiction is visible: after all, numbers are infinite in the real world! And one more thing: the Universe will expand and cool down, which interferes with the process. In general, it is not clear who to believe. I don’t know what to choose yet, but by commenting, you will help me choose the difference between this.