As per Lord Krishna in Bhagavad Gita the present cosmos came into being when Brahman, God Almighty exploded self with a big bang. The big bang theory is not a hypothesis but an actual fact of cosmic life. In absence of big bang theory the present cosmos just could not exist, create!
The principle behind formation of present cosmos can be understood by going in past. Let us go in time just before occurrence of big bang. This is the stage when old cosmos is rapidly dissolving into a large black hole. This singularity is what resulted in formation of present cosmos.
The present visible cosmos vanishing into a single black hole and becoming the size of half a thumb is inconceivable, unimaginable. But this is what actually happened just before the big bang. The dissolution of entire old cosmos (called Pralaya in Hinduism) reduced to size of half a thumb, and what is this half a thumb?
In absence of dross impurities within all souls’ atmans in cosmos occupy the volume of half a thumb. This magnanimous primordial cosmic energy, size of half a thumb… collective power of all pure souls’ atmans in cosmos is what we called God Almighty, Brahman Parmatman in Hinduism.
This magnanimous primordial energy unable to contain itself for long in its prime pure state again explodes with a big bang giving rise to a new cosmos, a totally new journey of life. An explicit explanation of origin of cosmos exists in Upanishads (independent spiritual treatises).