Theoretically, you are right. We haven't proven that we aren't currently flying off into space right now, but based off of what we can observe and know, it's very likely that we are orbiting our galaxy (not necessarily SagA, but, rather, a sort of galactic barycenter). It's not a fact in the scientific/mathematical sense of the word, but the probability of any other explanation is so low that we can treat it as such and form our assumptions around it. That's how most high-level science and mathematics work though. Many things that we treat as facts or proofs are actually just theories and very educated guesses, but they work very well for us and help us innovate and learn about the world around us; sometimes we don't have to prove something for it to be useful to us.