Unity uses backface culling. Because the normals (arrows that indicate the front of a face) are turned to the outside, you can't see the inside. You only modelled one side of your cube and no inside.
You could think of modelling an inward cube as well inside your cube, or you could turn your backface culling off. But this would give problems with texturing, because bricks will be on the inside AND the outside when you put a texture on your walls.
