Seated on His chariot with Arjuna, Kṛṣṇa proceeded north, crossing over many planetary systems. These are described in Śrīmad-Bhāgavatam as sapta-dvīpa. Dvīpa means "island." These planets are sometimes described in the Vedic literature as dvīpas. The planet on which we are living is called Jambūdvīpa. Outer space is taken as a great ocean of air, and within that great ocean of air there are many islands, which are the different planets. On each and every planet there are oceans also. On some of the planets the oceans are of salt water, and on some of them there are oceans of milk. On others there are oceans of liquor, and on others there are oceans of ghee or oil. There are different kinds of mountains also. Each and every planet has a different type of atmosphere.
Kṛṣṇa passed over all these planets and reached the covering of the universe. This covering is described in Śrīmad-Bhāgavatam as great darkness. The material world as a whole is described as dark. In the open space there is sunlight, and therefore it is illuminated, but in the covering, because of the absence of sunlight, it is naturally dark. When Kṛṣṇa approached the covering layer of this universe, the four horses which were drawing His chariot—Śaibya, Sugrīva, Meghapuṣpa and Balāhaka—all hesitated to enter the darkness. This hesitation is also a part of the pastimes of Lord Kṛṣṇa because the horses of Kṛṣṇa are not ordinary; it is not possible for ordinary horses to go all over the universe and then enter into its outer covering layers. As Kṛṣṇa is transcendental, His chariot and His horses and everything about Him are also transcendental, beyond the qualities of this material world. We should always remember that Kṛṣṇa was playing the part of an ordinary human being, and His horses also, by the will of Kṛṣṇa, played the parts of ordinary horses in hesitating to enter the darkness.
Kṛṣṇa is known as Yogeśvara, as stated in the last portion of the Bhagavad-gītā. Yogeśvaro hariḥ: all mystic powers are under His control. In our experience we can see many human beings who have yogic mystic power and who sometimes perform very wonderful acts, but Kṛṣṇa is understood to be the master of all mystic power. Therefore, when He saw that His horses were hesitant to proceed into the darkness, He immediately released His disc, known as the Sudarśana cakra, which illuminated the sky a thousand times brighter than sunlight. The darkness of the covering of the universe is also a creation of Kṛṣṇa's, and the Sudarśana cakra is Kṛṣṇa's constant companion. Thus He penetrated the darkness by keeping the Sudarśana cakra before Him. Śrīmad-Bhāgavatam states that the Sudarśana cakra penetrated the darkness just as an arrow released from the Śārṅga bow of Lord Rāmacandra penetrated the army of Rāvaṇa. Su means "very nice," and darśana means "observation"; by the grace of Lord Kṛṣṇa's disc, Sudarśana, everything can be seen very nicely, and nothing can remain in darkness. Thus Lord Kṛṣṇa and Arjuna crossed over the great region of darkness covering the material universes.