NASA's "Astronomy Picture of the Day" goes back more than a decade. Not all the pictures are free to use - the majority are copyrighted - but anything created by NASA and government orgs is in the public domain, and some of them are gorgeous. Each picture has credits underneath; if it doesn't say copyright or license, it's free to use.

https://apod.nasa.gov

A few pictures:

• Doomed Planet 
• Saturn at Night
• Omega Nebula

There are also videos, and I'm pretty sure those run on the same system - if they just say "credit," they're made by the US gov't and are in th e public domain. But most of them are copyrighted.

This is a fantastic resource, thank you for posting it!

I do want to comment that this jam is being very informally judged by a contracted NASA engineer who I happen to be related to. He has asked that I remind folks very gently of the following clause:
"If the NASA material is to be used for commercial purposes, including advertisements, it must not explicitly or implicitly convey NASA’s endorsement of commercial goods or services"

So, like, yes. Definitely use those images! But do make sure if you're leaning especially hard on the theme of NASA or you've got the meatball logo all over your game or something that you put a little blurb on your cover page that you are an independent production not endorsed by the National Aeronautics and Space Administration. My dad's not a rat or anything, he's not even American, but it's good manners, yanno?