Hinduism is a vast religion and probably the oldest one on earth and since every religion has its own theory of birth or creation of the universe, it is apparent that the oldest religion with oldest mythology would probably have one as well. Though in Hinduism this theory is very vast and interesting and has many perspectives in accordance with many sects of Hinduism; there is one entity which is common among all the sects of Hinduism who is the creator of the universe whom we know as Lord Brahma or the creator.
It was the time when time itself was unborn and nothing-literally nothing-existed moving or static. The whole space was asleep and Lord Narayan was at the centre of it-resting on Anant Sheshnaag when from his naval(naabhi kamal), Lord Brahma was born. When Lord Brahma asked Lord Narayan about the cause of his birth then he was told that it is his duty to create a universe. Lord Brahma obeyed the Lord and obliged and hence the universe came into existence.
There is also a theory of Hiranyagarbha i.e. the golden egg or the golden womb. It is believed that when none existed, the supreme soul or brahman existed and then he created cosmic waters and established a seed in it which turned out to be a golden egg or Hiranyagarbha. The Super Soul entered it and then it broke into two giving birth to the creation of the universe and from the same cosmic golden egg was born Lord Brahma.
Now lets talk about the life of Lord Brahma and what we know about him.
Lord Brahma is actually believed to reside in Brahma lok where he lives with his consort goddess Brahmani or goddess Saraswati. He with his four heads has created four Vedas and his first creation was the four kumaras- Sanaka, Sanandana, Sanatan and Sanat who refused to lead the creation as they were completely engrossed in Lord Vishnu/Lord Narayan and were celibates. His life span is that of 100 ‘Brahma-years’ which is much more when compared with our 100 years. His few seconds involves many years for humans on earth.
The life span of Lord Brahma is 100 ‘Brahma-Years’. One day in the life of Brahma is called a Kalpa or 4.32 billion years. Every Kalpa creates 14 Manus(Manu is first male human created by Brahma for birth of humans on earth whose wife Satrupa is first female to give birth to humans on earth. They are like Adam & Eve of Hindu Mythology) one after the other, who in turn manifest and regulate this world’s 4 yugas(yugas=ages)-Satyug(golden age), Tretayug(silver age), Dwaparyug(bronze age), and Kaliyug(iron age in which we are living). At the end of it i.e., at the end of Brahma’s life span, the destruction by Lord Shiva takes place after which Lord Narayan again rests and Brahma gets dissolved in him.
So this is how this cycle takes place again and again and Brahma gets birth for the creation and gets dissolved in Lord Narayan at the end and this is how our universe comes into existence and then gets dissolved all over again with the dissolution of our creator-Lord Brahma.