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.