Recently I came across Alex Patterson’s Spacescape program. It creates space skyboxes by building up layers of stars, billboards (special stars, galaxies or other external images) and noise (for gas and nebulae). These can then be exported as a standard 6 skybox images each up to 4096×4096 resolution. All this under the permissive MIT License. Definitely worth checking out if you are developing a space-based project.
This could be very useful in Concealed Intent. However, there are a couple of issues to be noted when importing the resulting images into a Unity skybox. For more details see the Unity Texture documentation