Question

how did we know about big bang?

Answer 1

If most galaxies are moving away from us, it means that the Universe is expanding. If the Universe is expanding, then in the past it must have been much smaller. Go back far enough, and there was a moment when all the matter in the Universe was packed into a point and expanded outwards. That moment was the Big Bang.

Answer 2

To put it simply, the Big Bang theory suggests that we live in a perpetually expanding universe that is approximately 14 billion years old, and in the first moments of existence, all of the matter in the universe burst forth from an infinitely dense point called a singularity.