The bakground I used comes from a scientific project of astrophysics I am working on. Surprisingly (or maybe not) scientific graphs can give very beautiful images, as well as for background as for patterns to texture objects; I'd wanted to use one in a render for a long time...
This backgroung is in fact a part of a simulated map, showing galaxy clustering in the Universe. I first created a 360 map with the healpy python package, that I converted into a HDRI by using some tricks in Blender. The second additionnal image uses this HDRI.
But finally, I found it didn't look the best in my scene, so I changed my mind and I used it as a "flat image" background in GIMP. Then, I searched for the best detail part to use as the background.

I hope you'll enjoy it!