Ofcourse, the more complex a theory is, the harder it is to first understand it, and second disprove it.

The simple fact that Darwin's theory has been uncontested (successfully, at least) for close to 150 years, largely speaks in it favor ofcourse.
Certain points of it have been refined or been added upon, but, that I know, nothing of his theory has been proven to be false, or even unlikely.

It's possible though that in, say, 300 years, our knowledge has become vast enough to be able to disprove it. My bet would be on that it wont, though. Maybe it's indeed just a piece of a larger puzzle, but then it's likely the piece that sets us on the way to solving it completely.