Of course he considered it a theory.[1] It was a new idea. He had a pile of evidence for it (yes, empirical evidence) but it still needed to be looked at by other scientists and the evidence assessed.

I don't know what you mean by "not empirical", but if you mean that Darwin just wrote "hey, here's a cool idea" and didn't have empirical evidence for it then you're just flatly wrong. If you mean that now we don't have empirical evidence for it then either you've been terribly deceived by others or you're lying. (Probably the former. My condolences.)

Something can be a theory and also factual. And there can be a theory of something that is a fact. If you think "X is a theory!" is some sort of refutation of people who regard X as true, then you are 100% wrong about that.

The usual progression goes like this. Someone comes up with a theory (in the vernacular sense). Say, Darwin's theory of how living things got to be the way they are. Some theories are more or less correct from the outset. Some are just plain wrong. Most are somewhere in between. If the theory seems worth taking seriously, other people try to work out its consequences in more detail, and go looking for evidence for or against it, and refine the details. When the theory is all wrong, it will hopefully get knocked down as this happens. (Examples: "cold fusion", Lamarckian "inheritence of acquired characteristics".) When the theory is basically right, the wrong bits will get corrected (e.g., Darwin expected inheritance to be a sort of "mixing" process, which isn't really right and produces some wrong intuitions) and new ideas will be brought in (e.g., Darwin didn't know about genes), and as repeated investigation doesn't refute the underlying ideas it becomes increasingly implausible to reject them.

Today's understanding of evolution isn't identical to Darwin's. He made some mistakes and some wrong guesses and there was a lot he didn't know. But we have a lot more evidence than Darwin did that populations of living things evolve over time, that a major cause of the change they undergo is natural selection, that even very different-looking living things have common ancestors, etc.

Darwin had a "theory" in the vernacular sense: an idea that seemed to explain a lot of things and might or might not turn out to be right. It was a good enough idea that scientists after Darwin put a lot of effort into investigating and refining it. We now have a "theory" in the "grand scientific edifice" sense, and while details will continue -- ahahaha -- evolving, there's little scope for reasonable people aware of the evidence to doubt that its central ideas are facts.

[1] Even in the "vernacular" sense. It's true that sometimes "theory" means "grand scientific edifice, well evidenced and elaborated with mathematical sophistication", but it doesn't always, and when Darwin called evolution by natural selection a "theory" he didn't mean that.

A theory is the pinnacle of scientific achievement - a complete series of observations, equations, processes etc. that come together to describe a phenomenon.

There is nothing that we know with a higher degree of certainty then a scientific theory, such as the theory of evolution, the theory of general relativity, the theory of quantum mechanics, the theory of newtonian mechanics etc.

> The meaning of the term scientific theory (often contracted to theory for brevity) as used in the disciplines of science is significantly different from the common vernacular usage of theory.[4][note 1] In everyday speech, theory can imply an explanation that represents an unsubstantiated and speculative guess,[4] whereas in science it describes an explanation that has been tested and widely accepted as valid. These different usages are comparable to the opposing usages of prediction in science versus common speech, where it denotes a mere hope.

Please take a minute to consider this and realize that you should never repeat what you just said.

Gravity is also a theory.

"There is a notable difference between the opinion of scientists and that of the general public in the United States. A 2009 poll by Pew Research Center found that "Nearly all scientists (97%) say humans and other living things have evolved over time – 87% say evolution is due to natural processes, such as natural selection. The dominant position among scientists – that living things have evolved due to natural processes – is shared by only about a third (32%) of the public."

-- https://en.wikipedia.org/wiki/Level_of_support_for_evolution

Um, that's how science works. It's an effort to explain the world. Everything is a theory, nothing is final. Some theories (evolution, gravity) are more credible/developed than others.

There's been a lot of evidence accumulated for it since Darwin's time.