Bhagavad Gita: Chapter 8, Verse 18

अव्यक्ताद्व्यक्तय: सर्वा: प्रभवन्त्यहरागमे |
रात्र्यागमे प्रलीयन्ते तत्रैवाव्यक्तसञ्ज्ञके || 18||

avyaktād vyaktayaḥ sarvāḥ prabhavantyahar-āgame
rātryāgame pralīyante tatraivāvyakta-sanjñake

avyaktātfrom the unmanifested; vyaktayaḥthe manifested; sarvāḥall; prabhavantiemanate; ahaḥ-āgameat the advent of Brahma’s day; rātri-āgameat the fall of Brahma’s night; pralīyantethey dissolve; tatrainto that; evacertainly; avyakta-sanjñakein that which is called the unmanifest


BG 8.18: At the advent of Brahma’s day, all living beings emanate from the unmanifest source. And at the fall of his night, all embodied beings again merge into their unmanifest source.


In the amazing cosmic play of the universe, the various worlds (planes of existence) and their planetary systems undergo repeated cycles of creation, preservation, and dissolution (sṛiṣhṭi, sthiti, and pralaya).  At the end of Brahma’s day, corresponding to one kalp of 4,320,000,000 years, all the planetary systems up to Mahar Lok are destroyed.  This is called naimittik pralaya (partial dissolution).  In the Shreemad Bhagavatam, Shukadev tells Parikshit that just as a child makes structures with toys during the day and dismantles them before sleeping, similarly, Brahma creates the planetary systems and their life forms when he wakes up and dismantles them before sleeping. 

At the end of Brahma’s life of 100 years, the entire universe is dissolved.  At this time, the entire material creation winds up.  The pañch-mahābhūta merge into the pañch-tanmātrās, the pañch-tanmātrās merge into ahankār, ahankār merges into mahān, and mahān merges into prakṛitiPrakṛiti is the subtle form of the material energy, Maya.  Maya, in its primordial state, then goes and sits in the body of the Supreme Lord, Maha Vishnu.  This is called prākṛit pralaya, or mahāpralaya (great dissolution).  Again, when Maha Vishnu wishes to create, He glances at the material energy in the form of prakṛiti, and by His mere glance, it begins unfolding.  From prakṛiti, mahān is created: from mahān, ahankār is created; from ahankār, pañch-tanmātrās are created; from pañch-tanmātrās, pañch-mahābhūta are created.  In this way, unlimited universes are created.

Modern day scientists estimate that there are 100 billion stars in the Milky Way.  Like the Milky Way, there are 1 billion galaxies in the universe.  Thus, by estimation of scientists, there are 10^20 stars in our universe.  According to the Vedas, like our universe, there are innumerable universes, of differing sizes and features.  Every time, Maha Vishnu breathes in, unlimited universes manifest from the pores of His body, and when He breathes out, all the universes dissolve back.  Thus the 100 years of Brahma’s life are equal to one breath of Maha Vishnu.  Each universe has one Brahma, one Vishnu, and one Shankar.  So there are innumerable Brahmas, Vishnus, and Shankars in innumerable universes.  All the Vishnus in all the universes are expansions of Maha Vishnu.