Krishna gives a description of the eternal sky in Bhagavad Gita: “That abode of Mine is not illuminated by the sun or moon, nor by electricity. And anyone who reaches it never comes back to this material world.” (Bg. 15.6)
This verse gives a description of that eternal spiritual sky. We have a material conception of the sky, and we think of it in relationship to the sun, moon, stars and so on, but in this verse the Lord states that in the eternal sky there is no need for the sun nor for the moon nor fire of any kind because the spiritual sky is already illuminated by the brahmajyoti, the rays emanating from the Supreme Lord.
Krishna resides eternally in His abode in the spiritual sky on a planet called Goloka, yet He can be approached from this world, and to this end He comes and manifests His real form in Vrindavan and performs wonderful pastimes there.
When Krishna manifests this form, there is no need for our imagining what He looks like. To discourage such imaginative speculation, He descends and exhibits Himself as He is, as Syamasundara. Unfortunately, the less intelligent deride Him because He comes as one of us and plays with us as a human being. But because of this we should not consider that Krishna is one of us. It is by His potency that He presents Himself in His real form before us and displays His pastimes, which are prototypes of those pastimes found in His abode.
In the effulgent rays of the spiritual sky there are innumerable planets floating. The brahmajyoti emanates from the supreme abode, Krsnaloka, and the anandamaya-cinmaya planets, which are not material, float in those rays. The Lord says, “One who can approach that spiritual sky is not required to descend again to the material sky.”