then why is the universe expanding?
The universe isn't "expanding". It's infinite. Something that is infinite can't expand.
Why would anyone want to proof that?
It's not like it actually matters what happened x million/thousand years ago
Okay, let's do this again.
A. So you're telling me you can't prove it happened? Okay then.
B. What happened in the past can determine the future. That's why it matters what happened X million/thousand years ago.