I heard this often again today, and I would like to correct it. The scientific use of the word “theory” is very different from common usage.
In common usage, theory means something that is speculative, and is often contrasted against “practice.”
The United States National Academy of Sciences defines scientific theories as follows:
The formal scientific definition of “theory” is quite different from the everyday meaning of the word. It refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence. Many scientific theories are so well established that no new evidence is likely to alter them substantially. For example, no new evidence will demonstrate that the Earth does not orbit around the sun (heliocentric theory), or that living things are not made of cells (cell theory), that matter is not composed of atoms, or that the surface of the Earth is not divided into solid plates that have moved over geological timescales (the theory of plate tectonics)…One of the most useful properties of scientific theories is that they can be used to make predictions about natural events or phenomena that have not yet been observed.
Scientific theories are improved or replaced by better theories as more evidence is discovered, therefore increasing in accuracy. When someone says, “That is only a scientific theory” in a way to try to dismiss it, you can know that they do not understand what they are talking about.