Mahabharat is not just a story, but the detailed account of a event occured in the past. The few points are noted below that indicate a few differences in what is 'reality' and 'myth'.
1. It has been written in the epic from time to time that Mahabharat is a "itihas" which exclusively means "thus occured". The words "Puraan" and "Itihas" were specifically coined by the Arya people to catagorize the "ancient" and "recent" events. Both the words denote history that has occured at different times.
2. It is mentioned in Aadiparva, Adhyaya 62 that the annals of the Bharat-Dynasty are recorded in the work.
3. It has been clearly stated in the Aadiparva, Bheeshmaparva etc. that this is "itihas". If the intentions of the writer were to write a poem or a work of fiction, he would have stated it to be a "mahakavya" or "katha".
4. It would to absurd to say that the Mahabharat is not a "itihas" due to its poetic nature. It was a custom in those days to write everything in poetic form.
5. Ved Vyas had decided to write down the "itihas" even before the initiation of the Mahabharat War. Therefore during the course of the War, Vyas meticulously noted down all the possible details. If it were a work of fiction, why would a person like Vyas want to fill his work with such minute and unnecessary details ?
6. A number of dynasties with their lond lineage of kings have been presented in the work. More than 50 kings from King Barhi to the Pandava King have been recorded. Additional information about the King, his wife, his scions, his relations, etc. have been accounted in great detail. If it were just fiction, only 4-5 kings would have sufficed to build the story on. Then why such mind-boggling details ?
7. The dynasties recorded in the Ramayan and the Mahabharat concur without a difference. Even the relations between different kings and their dynasties in both the great "epics" match with each other. If both were mere "epics" written by two entirely different at two different times, why would everything match even upto minor details ?
Mahabharat is of a later date than the Ramayan. Why would the author of the Mahabharat borrow the same ideas and characters as those of the author of Ramayan ?
8. Usually, the story of any "Maha-Kaavya" circulates about one or two main characters. If this were the case with Mahabharat, who would then be considered the "hero" of the drama ?
9. Many events mentioned in the Ramayan and Mahabharat are the same. Eg.: The mother of (latter) King Sagar was poisoned by his step-mother so that her child would be aborted. But the child was born nevertheless, who was therefore named Sagar.
10. The cities established by certain kings has been noted in detail.
11. All the characters in the "epic" are well-portrayed. Even single facet of their character and important events in their life have been recorded. Are such detailed accounts important in a "Maha-kavya"?
12. The weapons mentioned in the Ramayan and the Mahabharat are somewhat same. Infact, some weapons in the Ramayan are not mentioned in the latter "epic". (eg. Soorya'stra, Yamya'stra, Shoolva'stra..etc.) [ Considering the true occurance of the two great events, the above mentioned weapons might have disappreared in the era in-between the two events took place].
13. If it were a poetic fiction, such comprehensive account of the events on the battle-field would'nt have been given. For a poem, it is far-fetched. It will only serve the purpose of boring the reader to death!
14. The description of such myriad of characters is astonishing. It is impossible for one single-mind to be the genesis of that number of personality-types. It could only be true if the Mahabharat is the recording of a real-life drama.
15. The time and place of events have been accurately recorded. All such recordings are redudant for a "Maha-kavya".
16. Not much poetic description of the flora-and-fauna is given. Such description in ornate language is only used in fictional works and not while recording history.
17. Vyas mentions to have written this "itihas" after the death of King Dhrutarashtra. Why would he write so ? Did Shakespeare say that he wrote "Hamlet" after the death of Hamlet himself ?
18. The Greek historian Megasthenes has stated that Chandragupta Maurya was the 138 King in the lineage of Shri Krishna. This means that Shri Krishna did exist in the bygone era and that Mahabharat did really occur.
19. It was a custom to keep a track of the Kings lineage. The Chinese traveller confirms the above. Manahbharat being a true account of a occured War, such lineages are seen to be recorded.
20. Archaeological excations has discovered the submerged city of Dwaraka. This is the same Dwaraka as mentioned in the Mahabharat. [ The city of Dwaraka has been reckoned to have drowned in between 2000-3000 B.C.]
21. The astronomical recordings in the Mahabharat "epic" and other scriptures (Bhagwat), given the correct positions of the planets and stars during that time. How could a work of fiction be proved using mathematical tools ?