Suppose we develop a new mobile or a technique with which we can communicate with a human foetus in the mother's womb.
We tell him/her: in a few days/weeks you are going to come into a beautiful world. You will see green plants, fragrant flowers, huge mountains, blue sky, rains, rainbows, etc.
The baby (foetus) replies, this is the age of scientific advancements. Everything you say should have some scientific explanation for it. Let me explore and experiment and use all the scientific tools to see if all that even exists.
Then he starts exploring inside the womb to look for any evidence for the existence of this "beautiful world". Will he find any evidence for it?
He will say, I have used all available scientific means, and all I could find was dark, tight packed walls, some liquid and nothing else.
So according to the established Rules of Empirical, Stable, Demonstrable Protocol, Science says there is no such world.
Now how will you explain the existence of this world to someone who has not yet arrived here? Similarly, we cannot prove the existence of the Hereafter, Jannah and Jahannum using the scientific tools. All we can do is to rely on those who have seen it, and to believe in the Words of the One who created us.
Allah Subhanahu wa Ta'ala has shown our Prophet Sallallahu 'Alaihi wa Sallam the punishments of the Fire and the Bounties of Paradise. We are blind with respect to life hereafter, and our Prophet is the one who has seen it. Now its the duty of the blind to follow the one with eyes so as to save himself from falling in a ditch.