All games need good audio/visual resources. Read: images and music. Also, we are though from start to respect copyright, especially on internet where everything is at disposal. One way of getting starry background for our asteroid game is to find copyright free (public, copyleft, ...) images of stars filled backgrounds. Or, maybe even cooler way is to let computer generate it!
Thanks to my work colleague John E. I have been pointed in right direction.
Here is, for instance, link to create nebula images in procedural way - all online: http://wwwtyro.github.io/procedural.js/space/. For those who want to learn more - how it is done - code is in github - here.
After adding starry background, our game needs music, too. Ripping the latest currently popular song is illegal. And inappropriate - who saw/hear game with the latest hits as a background music. On the other hand, eighties 8-bit music might be exactly what you're looking for. So here is the link for python script that can compose your own songs:
If you don't like the first song - try it again, and again. Aside of proper, background 8-bit alike sound, that script produces creative song titles, too. For instance, did you hear the latest song called "A fish and puppies"? Or "An orange retort"?
Aside of music and background, you can even create your own sound effects online: http://jfxr.frozenfractal.com/. You can create sounds from zapping laser to picking coin.
So, grab a music you like, create background and add sound effects to your game and see you on the next session! Happy coding!