So a theory an explanation of the pattern created by the outcome of extensive experimentation in that area that the scientific community generally agrees upon as probable. Since you can never truly prove your hypothesis is correct, and a theory is the sum of many supported or not-supported hypothesis, you can say that there is indeed a chance that the theory is wrong and is not an absolute truth.

Since we don't actually know the bounds of our universe who's to say that the earth really isn't in the center? If it expands forever, what's the middle of infinity? anywhere? if that's the case then maybe we are in the center. But that's not what's been supported by scientific experimentation and observation.

A key aspect of an experiment is the ability to create a controlled environment in which certain variables are held constant and while others can be selectively manipulated to produce the corresponding results. A challenge aspect of truly describing nature in a scientific sense is that nature is not a controlled environment, and any attempt to create one removes that environment from the natural world, so we never would really know if the results are truly correct or it came from creating a controlled environment within nature.

This is why science has theories. Because we can never be sure with absolute certainty that specific explanations of the universe around us are true, all the data is looked at, argued about for a while, maybe do a few more experiments just to make sure, and if an overall consensus is made then the general scientific community will knowledge the theory. But they won't say it's true, merely supported by tons and tons of observations. I think this is where people get "just a theory" from, that or their confusing it with hypothesis.

tl;dr Theories themselves aren't absolute truths because scientists admit that they could very well be wrong and could be Earth is standing in the middle while everything else flies fast as fuck around us. We just don't see evidence to support that.