Sri Aurobindo translates Prashna Upanishad, 1st Question: “Lord, whence are all these creatures born?”
There are multiple parts to the answer provided by the Sage Pippalada. First he declares that the Eternal through concentration of conscious force created Energy and Matter: “The Eternal Father desired children, therefore he put forth his energy and by the heat of his energy (n.b. tapas), produced twin creatures, Prana the Life, who is Male, and Rayi the Matter, who is Female. ‘These,’ said he, ‘shall make for me children of many natures. The Sun verily is Life and the Moon is no more than Matter: yet truly all this Universe formed and formless is Matter: therefore Form and Matter are One.”
Rayi, the term used for Matter, also means “food”. Prana, the life-force, is, as we have seen in the Taittiriya Upanishad, considered to be the “eater of food”. The Sun is the source of the energy that creates and sustains Matter and Life. The conversion of energy into matter, and matter into energy, is therefore implied. Further implied is a concept that is actually a modern scientific concept being proven out by scientists of today — namely, that “all this Universe formed and formless is Matter.” The amount of visible matter in the universe is very small, but the universe reacts as if there is something other than visible matter. This is being called “dark matter”. We could understand that visible matter is the “formed” and dark matter is the “formless”. Others are exploring the idea of whether there is a subtle material “ether” as the ancient sages indicated, to help describe the phenomena they see in their measurements of energy and the movement of celestial bodies. Thus, the insight of the Upanishadic sage is being confirmed, similar to the insight that energy and matter are interchangeable, that energy creates matter. Finally, the insight of the Eternal, the All-Conscious, forming both energy and matter, is now becoming known to science as they follow the path of “matter is energy, energy is consciousness”.
The further Western science delves into the secrets of the universe and the origins of living beings, the closer they get to the spiritual truths as seen and experienced by the ancient sages.
There are additional subtleties to be understood here as well. In the Vedas and Upanishads, the Sun does not simply represent a physical body that radiates energy through conversion of matter, but also psychologically, the force of consciousness that creates the multiplicity we experience as the universal manifestation, the Vijnana. The Isha Upanishad describes this Sun as a power of awareness.
Sri Aurobindo, The Upanishads, Prashna Upanishad, pp.297-315