This is phenomenal and whimsical. I don't know Photoshop that well so I can't fairly say anything about your technique. The color choice is interesting, I think of the ocean instead of space until I see the red giants dotting the lower left corner. This piece makes me feel so small but at the same time I feel like I could reach our and disperse the nebula like mist with a wave of my hand. The transition between the darkest corner bottom left could transition a bit more subtly into the light of the nebula. Unless that is how such space formations are for I have little knowledge on the subject and possibly no say in how the light ought to transition into darkness.
First off, this piece certainly deserves the DD. The overall composition works great, the focus being about 1/3 from the right, and the colours are calm and serene. The stars look great, with realistic colours and a nice, not too random spread that still doesn't look deliberate. The nebula cloud itself looks awesome, the shape looks really natural and stands out well.
My only criticisms would be that the stars' visibility is not affected by the cloud, giving the impression that all the stars exist in a flat pane in front of the cloud. I would reduce the brightness of some stars where the nebula is, to give the impression that they're shining through a dense cloud of dust. This would perhaps improve the depth of the piece. Also, while the nebula cloud looks great, there isn't much going on around it, and I think some subtle variations of tone and colour (like in the bottom left, which looks great) would improve the piece. Using more purple-brown tones in the shadows of the nebula cloud might look good.
Overall though a stunning piece and definitely my new wallpaper. The technical skill and unity of composition really stood out for me, I would love to know if this nebula was 100% hand-painted or if some sort of fractal program was used as a base. Either way this piece is stunning. Awesome work!