How did scientist's came to know about big bang theory?

It's the oldest question in human history. Everyone's asked it, from philosophers and poets to mathematicians and physicists. Most scientists think that everything that we know and experience around us began at a moment known as the Big Bang, 14 billion years ago. read more

They came to know about it from the relic radiation that was left behind from the recombination phase after the big bang. Lighter elements such as hydrogen and helium were formed after sufficient cooling down of the universe, protons and electrons could form pairs and make a stable atom. read more

Scientists have also discovered a predicted thermal imprint of the Big Bang, the universe-pervading cosmic microwave background radiation. And we don't see any objects obviously older than 13.7 billion years, suggesting that our universe came into being around that time. read more

Einstein's model of Gravitation called The General Theory of Relativity allowed the construction of self consistent Cosmological Models for the first time. But General Relativity based Cosmological Models always came up with dynamic Universe (either expanding or contracting). read more