Theories, Hypotheses, and Guesses
The problem with this is that scientists use the word theory differently than when you have a theory about who stole your lunch at work. If you have a pretty good idea that it was Joe from accounting, but what you have is a hypothesis, not a theory.
One of the differences lies in experiment. A hypothesis is often untested, but just as often it has been tested, but not enough to be considered confirmed. Once a hypothesis has been tested enough to be cautiously labeled “true” (which takes a while), then it can be called a theory.
Another difference is that a Theory is a framework, it contains the explanatory models, the equations, the predictions, and all the experimental observations and data in one unified and coherent structure.
When scientists say Theory with a capital T, the theory has been tested, it has been tested a lot, and it has been confirmed. The Theory of Evolution is not a theory in the same sense as your “theory” about who took you lunch, it has been spectacularly confirmed at every available opportunity, and no piece of data has ever been found that conflicts with it. The same is true for other capital-t Theories, like the Germ Theory of Disease, Heliocentric Theory, the Theories of Special and General Relativity, and Quantum Theory. Saying “____ is just a theory” is essentially saying that despite decades, sometimes centuries, of experimentation that has yielded piles of data that all match these theories beautifully, we should throw them out.
All that it indicates is that the speaker is a clueless hack whose opinions on scientific matters mean absolutely nothing. Speaking that phrase is tantamount to saying “Look at me! I’m an idiot!”
So next time you start to use the word theory colloquially, stop yourself and use hypothesis instead, all the cool kids are doing it. And if you ever hear someone say “____ is just a theory”, kindly explain to them why they’re full of shit.