Scientists develop theories to be able to create a well-substantiated explanation for some thing/aspect of the natural/unnatural word which is based on facts that have been confirmed by many other scientists through different experiments and observation. A theory that is supported by facts are not a guess but a way to get closer to the truth. An example is Darwin’s theory of evolution. Evolution bring the gradual change of characteristics of an organism over a period of time. Darwin took an observation and experimented (I won’t go into full detail unless you’d like so) and developed his theory of ‘survival of the fittest’ (now known as natural selection) and of evolution, eventually composing a book which took him 20 years called ‘The origin of the species’ of you are interested in such things I’d recommend reading :)