How to set world sky environment for use in libgdx? I'm making a game using libgdx. 
I have to set a sky to my game , using models or another way. 
I looking how to make sky in blender .... I'm so confused. I need help.

For example I've made a sky using a sphere, and applied a skydome image. But in libgdx it is only visible from the outside, when I go into sphere it appears black. I mean the only outside picture seems
You would need to flip the normals, they are pointing outward, go to Edit Mode with your sphere selected. Select all A, from the mesh menu select Normals / Flip Normals



After that you normals should look like (display of normals can be activated by the highlighted button):