Since it’s a backdrop, you will need skills in compositing, matte painting, skybox creation, etc.
Your question is too open ended, like asking how to go about building a car.
Some tips:
- check out NASA for excellent and hi res imagery of space
- use hi res space imagery to comp your background in something like photoshop/after effects
- you can paint your own imagery if you are a good artist
good luck with your game!