Regarding how old the universe is, here is a small calculation according to Agni Purana:

Kaliyuga: 432000 years

Dwaparyuga: 2*Kaliyuga

Tretayuga: 3* Kaliyuga

Satyayuga:4*Kaliyuga

1chaturyugi: 1 Kaliyuga+1 Dwaparyuga+1 Tretayuga+1 Satyayuga= 4.32 million years

1 Manwantar: 56 Chaturyugi.

1 Kalpa= 14 Manwantar 

1 Day of Brahma= 12 Kalpa

100  years = age that Brahma lives to

50 years = Brahma's current age, or,

155.5 trillion years = the current age of this earth

When sages and some Brahmins do sankalpa (or daily jap), they had to say exactly what day of present the year of present chaturyugi, manwantar and kalpa we live in. Today, for example, is 28th chaturyugi of our manwantar. So, half of our kalpa is over.

Our current manwantar is called baibaswat manwantar.

(Please google to find out about it...I am just a curious fellow, not a professional readers of these old epics, so I may have erred in my interpretation.)

Now, there is a question about how come our ancestors could come up with these huge imaginary numbers while Christians believed god created men on his own image and the earth is 6422. (-91 / +155) years old, and was created over seven days

One reason is "zero". A Christian one thousand years ago couldn't write 1 billion easily . Their highest number was M(=1000 years), Writing M ten times (i.e. MMMMMMMMMM) would make it 10 thousand years. To write 1 million they had to write M one thousand times, and to write a billion, they had to write M one million times. Their imagination was limited by their inability to express it.

Going back to Mahabharata, yes, like i said before, there are some concrete proofs from that era. The war of epic proportion , which drew men from around south asia, happened. Like I said, the Katoch dynasty is one of the oldest dynasties in the world, almost 9000 years old, until it was vanquished in mid nineteenth century. Sansar Chanda, who fought in Kangada against Nepali, was 481st king of Katoch dynasty. Their 231st king Susharma build the fort of Kangada (our western border once). Susharma fought for Kaurav, and died in the war. His army were the ones who encircled Abhimanyu as he was sucked into Chakrabyuha.

The reason why Mahabharata is so popular (even its TV serial stopped the whole India and Nepal when it was being aired) is not hard to explain. First, it is full of intrigues. Second, it has love, war and history and everything in it. Third, it was rewritten over several centuries. When Panini standardized Sanskrit grammer 2500 years ago, they rewrote Mahabharata and other epics in new grammaer (but didn't touch vedas as they were too sacred to rewrite). Mahabharata has been translated into Nepali and all language, written and rewritten over verses, and stories. It has extensive subplots, each chapters are worth reading independently, it has moral lessons for kings and ordinary people so everyone can relate to it. More than anything, it told people that even kings had to be moral, responsible to their people, and that truth always wins.