Sri Aurobindo translates Taittiriya Upanishad, Brahmanandavalli, Chapter 6 (part 2): “The Spirit desired of old, ‘I would be manifold for the birth of peoples.’ Therefore He concentrated all Himself (Or, strength.) in thought, and by the force of His brooding He created all this universe, yea, all whatsoever existeth. Now when He had brought it forth, He entered into that He had created, He entering in became the Is here and the May Be there; He became that which is defined and that which hath no feature; He became this housed thing and that houseless; He became Knowledge and He became Ignorance; He became Truth and He became falsehood. Yea, He became all truth, even whatsoever here existeth. Therefore they say of Him that He is Truth. Whereof this is the Scripture.”
In The Life Divine, Sri Aurobindo describes what he calls “reality omnipresent”. The manifested world is not simply an illusion or some kind of lesser reality to be abandoned in favor of the unmanifested silent Absolute. The Upanishad declares that the entire manifestation is the self-creation of the Brahman for His own enjoyment. It is not separate from the Brahman; rather it is created of the substance of the Brahman and it embodies the Brahman. There is nothing else outside. Both the manifested universe and the unmanifest are One. The Brahman cannot be defined by any particular form, because the Brahman is all forms, and beyond all forms, it is both what exists and what is potential, the present and the future.
Sri M. P. Pandit notes: “It is out of Brahman, the Supreme Reality, that the All has issued. Why did it issue forth? Why did the Spirit, the One Brahman put out the Many? It is because of its own urge for a varied play of its self-delight. Blissful in its soleness of being, the Spirit would have Delight in a manifoldness of expression.”
Sri Aurobindo observes, in The Life Divine (as cited by M. P. Pandit): “The self-delight of Brahman is not limited, however, by the still and motionless possession of its absolute self-being. Just as its force of consciousness is capable of throwing itself into forms infinitely and with an endless variation, so also its self-delight is capable of movement, of variation, of revelling in that infinite flux and mutability of itself represented by numberless teeming universes. To loose forth and enjoy this infinite movement and variation of its self-delight is the object of its extensive or creative play of Force.”
This creation came about through the concentration of conscious force of the sole Existent, through tapas.
There are explanations of the Creation of the universe in religious and spiritual teachings from around the world. The Upanishads are somewhat unique in this regard in that they do not posit an external creator separate and different from the creation; rather the creation is One with the creator, made of the substance of the creator, and embodies the creator in its existence. Where others see duality, the Upanishad sees oneness.
Sri Aurobindo, The Upanishads, Taittiriya Upanishad, Brahmanandavalli, pp.265-274, M. P. Pandit, Upanishads: Gateways of Knowledge, pp. 109-182
Sri Aurobindo Studies 