Start with a base color. I like to go with really dark blues or purples. black is also good:

pick the base tone for your bg galaxy. Imma do a purple galaxy for this one.

use the line to outline what shape you want it to be!! It can be splotches, lines, wiggly, space gas is funky.

next, take whatever brush you like best for blending. I use firealpaca and like using the watercolor brush. these are my settings if you want it: and take that, and blend out your edges. I usually expand a bit of the color then use the dark color to blend. Whatever works for you is also good.

Don't worry if it isn't perfect! Gases in zerogravity kinda look like food color dropped into water. It's actually better if its chunky.

Then pick a highlight color! Whereever your highlights are will be where stars and galaxy clusters are most dense in your galaxy. Keep that in mind for later.

rinse and repeat the blend:

and then pick your final highlight, close to white. Blend. You might have to make your blending brush a lil smaller for tiny clusters.

Now for stars! Firealpaca has a particle brush that's good for this. these are my settings:

either on the base layer or a new one, paint your background stars. Don't worry about being perfectly around your galaxy colors.

Then, your foreground stars! Use a smaller brush to get in that density in your bright areas (the brighter the area, the more stars there should be), and then feel free to play with the size as you see fit.

Now you can stop here if you'd like. These next bits are optional, I'm just a nerd.

Now stars, when in photos taken outside the atmosphere, will refract light in 4 equal points, all at the same angle. It's not just a cartoon thing! why don't all the dots have that refraction? Cause most of them are GALAXIES, my dude. wild, right?

drawing a few refracting stars is super easy!

Slap on a lil cross, at an angle. on a layer above it, do a lil curved diamond that lines up with your cross, like above.

Fill it in! Don't worry bout making it clean. Unless you want to.

Gaussian blur that babeyyyy

Reposition as desired.

Copy and paste!

Congrats dude! You just did space art!