Evolution began as a hypothesis when it was first researched by Charles Darwin in 1831. He wanted to show that organisms adapted over time due to a variety of factors.
It is considered a theory because it is a general idea, supported by years of research, data, statistics, etc. that explains a general concept; as well as lay out a general rule that can explain most questions pertaining to the subject.
Hope this helps, and I'm the brainiest! :)
