Mahabharata in the words of Vyasa is a fiction work. The Story of Bharata is his family story. He presents this story to the world for the betterment of people. He realises the importance of such a story to be told since he foresees the mankind behavior and pain and suffering they undergo. He thinks that by reading/listening to his epic people can change their behavior for good and lead a very happy and prosperous LIFE.
The EPIC probably fall between 8th and 9th Centuries BCE (from wikipedia).
Vyasa executed the compilation of the Bharata, exclusive of the episodes originally in twenty-four thousand verses, each verse is a couplet. This 24,000 verses was the beginning of the Mahabharata. From this shorter version, the great EPIC Mahabharata was built.
Afterwards, Vyasa composed an epitome in one hundred and fifty verses, consisting of the introduction with the chapter of contents. After that he executed another compilation, consisting of six hundred thousand verses. Of those, thirty hundred thousand are known in the world of the Devas; fifteen hundred thousand in the world of the Pitris: fourteen hundred thousand among the Gandharvas, and one hundred thousand in the regions of mankind.
This entire compilation was first taught to Suka – Vyasa’s Son and afterwards Vyasa gave it to other disciples who were possessed of the same qualifications as that of Suka.
In this world they were recited by Vaisampayana, one of the disciples of Vyasa, a man of just principles and the first among all those acquainted with the Vedas. Vaisampayana at the Snake-sacrifice yagna of the Royal Sage Janamejaya and Parikshit recites the full Mahabharata ( 100000 Verses) for the benefit of Mankind.
Ugrasrava, the son of Lomaharshana, surnamed Sauti knows the entire 1000 verses by heart and tells this story to a group of Sages in the forest of Naimisha.
This is how, we get to learn about Mahabharata, from Sauti.