[tryp4vps]

..... My other question is if scientists say "Energy can neither be created nor be destroyed", then how does big bang happen? I mean, the "bang" itself released such a huge amount of energy that the universe came into existence. If this is something that comes up as a question, then why is the statement "Energy can neither be created nor be destroyed" still valid? 

The theory of "Energy can neither be created nor be destroyed" applies only to "human-sized" world.

In quantum world, everything behaves differently than the human-sized world, and follows the so-called uncertainty principle. The uncertainty principle says the creation of particles out of nothing is possible. That is how the term quantum fluctuation comes from.