The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Prior to that moment there was nothing; during and after that moment there was something: our universe. The big bang theory is an effort to explain what happened during and after that moment, there are many theories to explain the beginning of this universe, Big Bang is the most popular theory. Many of you may have been wondering about this is Big Bang the best explanation we could give? Wouldn't the Big Bang theory be considered a hypothesis and not a theory?
Well, I disagree with that. As for now, there is a lot of evidence in favor of the Big Bang Theory. I'd place some major evidences supporting the theory. If I miss any my apologies and please let me know.
First of all, we are reasonably certain that the universe had a beginning.
Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called Hubble's Law. This observation supports the expansion of the universe and suggests that the universe was once compacted.
Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. discovery of a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for.
Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins.
The expansion of the universe implies that at some time in the past everything must have been a lot closer together and hotter, which sounds a lot like the Big Bang to me.
In GR (General Relativity), there is a theory called the singularity theory which can be used to prove that there must have been a singularity (ie. a Big Bang) at some point in the past for every possible way we know to describe the Universe. There is probably more evidence than these, before I move on to the theory.