Science provides accounts of the world, past and present. It produces scientific theories. Scientific theories are ways of understanding what we can SEE in the world. They are based on observation and evidence.
Developing new theories involves new ideas. Often the first ideas are just suggestions. A suggestion that can be tested to see if it is true is a scientific suggestion, or hypothesis.
Scientific theories often incorporate scientific models, though theories and models are not the same things. Theories often start off as cautious scientific theories, but once they have been tested many times they can become established scientific theories.
Big Bang theory is an important scientific theory concerning space and everything in it, including us.