There are several options that allow you to define the appearance of empty space around your scene. This listing is not exhaustive, since any material can be used as the background material, however we'll most list the commonly used settings here.
Constant colour background
Illuminates scene with a uniform environment light.
Sun & sky
Indigo comes with a Sun and Sky environment that realistically depicts the sky. Changing the sun's direction creates time-of-day effects: a low angle creates a sunrise/sunset with correctly coloured sky and brightness.
The classic Sun & Sky model.
The following options are available for the classic model:
Turbidity: The turbidity defines the haziness/clearness of the sky. Lower turbidity means a clearer sky. Should be set to something between 2 and ~5.
Extra Atmospheric: Removes the sky and renders only the sun. Good for renders in space.
Since Indigo 3.2, there is also a new "captured" sky model, which is actually simulated by Indigo and captured to disk with the distribution.
The captured Sun & Sky model.
Illuminates scene with an environment map, which usually is a high dynamic range (HDR) image.
Indigo can load HDR environment maps in three formats, EXR (file extension .exr), RGBE (file extension .hdr) and a raw data format (extension .float, a simple format exported by the HDR Shop program); the environment map must either be in spherical format or equarectangular.
There are a number of setting available currently for environment maps. The emission values should usually be quite high, approximately 10^7 to be similar to the sun's brightness. The "Advanced Mode" checkbox ticked gives the users more options (full control over the image including Gamma and Texture Mapping modes), while disabling it reveals easy to use controls for rotating and tinting the environment map.