Evolution is a fact. That's because there are countless shreds of evidence that leads to the explanations to why the living world behave in its manner. A simple analogy is the breeding of dogs. Prior to 10,000 years ago, our ancestors came into contact with wolves. Some of these wolves were then domesticated and as the society progressed humans begin to selectively breed and produce multitudes hybrids of dogs. It's a mind-boggling feat if we look at the varieties of dogs today and to think that they were once wolves genetically altered by man to become so many types of canines.
The same things happen in the agriculture sector. It is hard to imagine that cabbage, Brussel sprouts, cauliflower and broccoli do not exist in the wild. They are all the products of human ingenuity. As a fact, all of them are of the same plant origin. We also see that apple and other fruits like durian went through evolutionary changes through mortal intervention.
The very same things also happen in the natural world. In this case, there is no human involvement required. The living things went through the process of procreation through natural selection. Unlike selective breeding, natural selection takes a long and gradual process. We are talking about in term of millennia if not hundred of million of years for species to split into new forms of species. What scientists have to work with now are fossils that create a snapshot of that process. It is difficult to observe in the wild the gradual transformation that takes place. That is however made possible in a confined environment of the lab by studying bacteria and microorganisms that can split into several hundred of generations in a short period of time.
So to say that Theory of Evolution is "just a theory" is very inept and almost childish. The problem lies perhaps in the semantic of the word theory itself. In our everyday usage, a theory could mean an assumption or a hypothesis. However, in science, the usage of the term is more stringent. Here it requires the collection of data which establishes the validity of the hypothesis as the foundation of the Scientific Method. The data is then compared to the hypothesis. The sample must be large enough and consistent for the hypothesis to be accepted. If there are discrepancies in the sampling, the hypothesis needs to be reevaluated or debunked. After going through the laborious process of assessing and reassessing then only a hypothesis qualifies as a theory. Hence, a theory in science is a model of observation based on facts. The Theory of Evolution is better off labeled as The Theory of The FACTS of Evolution!
While a whole lot of us will admit we don't know the Strings Theory in Physics, for example, many will not hesitate to give their piece of mind when comes to evolution. Turns out to be evolution may be the most misunderstood yet most talked about theory of all.