Create a document of size 750 by 550px.
Use Gradient tool with foreground color #2B2B2B and background #000000, start a linear gradient from the bottom-left.
Create a new layer on top of the Background. Set the foreground color to #000000 and background to #FFFFFF.
Go to Filter > Render > Clouds. Set the blending mode of this layer to Color Dodge.
Use Soft Eraser tool of size 300px, remove some parts of the clouds as shown in the diagram.