Is it a theory or is it a fact? well, its both.
In science, theory has a different meaning than when you or i use it. when we use it, we mean theory as in a guess, a hunch, a hypothesis. but in science, a theory is the explanation of the facts.
In science, theory is the highest status an explanation can have. a theory never becomes a fact in science. it can not be proven, it can only be disproven.
you also have other theories, such as say the theory of gravity. while its a theory, if you drop a hammer it will fall to the ground. so, hammers fall to the ground because of gravity, and that is a fact. however, the theory of gravity explains why it falls, namely it fell because of the gravity well created by the mass of the earth. so, you have the facts and the theory that explains the facts.
Likewise, with evolution, evolution is a fact. meaning things do evolve, we have observed speciation in larger animals, and we have observed microscopic life evolve in the lab. it is a fact that things such as nylonase did not exist and now do because of evolution. Then, as well as these facts, you have the theory of evolution, that explains why and how these facts are the way they are.
Im not sure if i have explained it too well there. The main thing to know, when you hear someone say “evolution is just a theory” is that this person probably does now know what theory means in this case. otherwise you would not say “just” a theory. the theory of evolution has more evidence for it that probably any other theory. so saying “evolution is just a theory”. its a bit like saying, buy my product, “its only a billion dollars”.
I wont mention religion much, as you say its not about that, but its worth noting, that its only really when there is religion in the equation, that people challenge evolution. you dont tend to see it for any other theories, like gravity, germs, the atom, and so on.
Is evolution a theory? yes.
is evolution a fact? yes
has evolution been proven? no
will it ever be proven? no
do theories ever become laws? no