Hypotheses vs theories
When someone says they have a theory, in everyday life, it could be completely crazy. I have a theory that my rabbit is being a jerk on purpose, which you may or may not think is completely nuts. But when someone has a theory in science, that means something different. If something is being called a theory in science, it has the weight of evidence behind it already. It’s only a theory because there may still be factors we don’t understand, but it has enormous and proven predictive power.
Take, for example, the theory of gravity. You might think that’s a given by now; we can observe it every minute of every day, and it’s proven consistent over the long term. There are equations describing gravity, and we can predict its effects. We know that when there’s a body of a certain size, we’ll observe a certain amount of influence on the part of gravity. That makes it a fact, right?
Well, in everyday terms, yes, it does. But in science, it’s still only a theory. There’s even ideas about modifying how we model gravity, so that we can better resolve questions about how the universe works and where it came from. While we can be certain that if we drop a ball on Earth it’s going to fall toward Earth (although you might think otherwise watching a ball come off Leigh Halfpenny’s boot), we can’t be certain that gravity works in the same way throughout the universe, in every situation.
That’s why it’s important to distinguish between theories and hypotheses. A hypothesis is a prediction about the world which can be tested: I hypothesise that if I go and pick ten dandelions and feed them to my (non-jerk) rabbit, she’ll prefer the larger ones. I can test that now, provided I can find dandelions, and I can repeat the test. I can even statistically analyse the results, and eventually, with enough data, I can come up with a full theory which explains everything I’ve seen by actually testing — and you can repeat my experiment and come to your own conclusions as well. (Please don’t feed my rabbits anything weird.)
Theories can be challenged with new data: that’s an important part of the scientific process. So when someone argues that evolution is “only a theory”, they’re not wrong. It just happens to be a theory which holds up against every test we’ve come up with. The evidence for evolution is stronger than my evidence that my blood type is O-positive; evolution has been tested again and again, while my blood type has only been tested twice, and I could have inherited A-type from my mother. Evolution is a theory which fits in with every observation we can make of the world around us (although there remain arguments about how exactly evolution operates). Any rival theory has to be even more convincing.
Post last edited at 17.16 CEST 22.04.2017: Rephrased final paragraph to remove references to natural selection.