Evolution and Big Bang is not without evidence. And theory, is not about just making assumptions. There is a huge amount of bodily evidence to support them. A good example is Albert Einsteins 'Theory of Relativity'. As theoratical as it might sound to you (this is not like your everyday theory of how your tshirt can end up at ur ex house) its application ranges from space travel to hi-tech communication. That is how solid the theory status in science is. It is not simply given to any sort of assumptions.
Theory
"denotes the most powerful status that an explanation can attain".They dont accept it without questioning, unlike what u believe. They constantly testing the validity of the theory in order to find the ultimate truth.
Of course Science has many branches and unlike what u believe, they contribute each other instead of contradicting. They are consistent in its laws and theory. A geologist can converse with a cosmologist on how the earth being shaped up. There is of course the things that they do not know yet. Science never claim they know everything. Thus this healthy community contributed to constantly question each others approach to the problem given. They seek out for evidence and vindicating their theory, like what Einstein did.
And theory is achieved through
"comprehensive frameworks for describing, explaining and making falsifiable predictions about related sets of phenomena based on rigourous observation, experimentation and logic" -
quoted from QualiaSoup 'Skewed views of science' - This workflow is used to all scientific community and its the backbone of it all. That is the relationship between studying the light behaviour travelling in the fibreoptic cables for high speed internet AND the theory of evolution and big bang.
And you are selective if u accept science when u use the internet but rebuke at evolution and big bang without bringing forward any argument to support ur doubts - because that is unscientific.