Actually a theory isn't necessarily what you think it is.

"Theory is a group of ideas meant to explain a certain topic, such as a single or collection of fact, event, or phenomen. Typically, a theory is developed through the use of contemplative and rational forms of abstract and generalized thinking."
Source: Wikipedia

Just in case you think that wikipedia is random people bullshit here's the Google definition

"a supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained."
Source: Google

Webster got anything to say?
"an idea or set of ideas that is intended to explain facts or events"
Source: Webster Dictionary

So before you say that it's just an idea, let me go head ahead and say that it's an idea by so many people that it pretty much is set in stone.

Didn't know if you noticed that last part.
The Big Bang theory. Almost everyone believes it.
The universal law of gravitation is a theory now go ahead and tell me that's incorrect.
Newton's law of motion is a theory

Some theories can be pretty much set in stone.
Of course someone can come along and say it's incorrect and give evidence why, although that's highly unlikely.

There are some ludicrous theories, like conspiracy theories, although the difference is how many people believe in them. Pretty much every musician ever uses the music theory.
I also disagree that everyone you ask will give you a different definition of music due to the fact that schools do teach the music theory, therefore people who took music classes may give you the same definition. Give or take a couple words.