I have a problem. When I add a cube or sphere to the scene they are transparent. How can I disable it? I tried adding materials, but it didn`t help and even if it would have helped it still a very dumb way of making standard objects non transparent.
I don't think they are transparent, they just don't have any light on them. Try adding a directional light to the scene.
The reason they look transparent is because they look like the skybox. They look like the skybox because there isn't a light in the scene, meaning the colour of the objects will be equal to the ambient light, which by default in Unity is tied to the skybox.