Q:

Which fallacy?  Explaining something based on a hypothesis

I am studying physics and have often come across the line of reasoning that goes like this:
"Phenomenon A is/happens this way because otherwise it would contradict the B principle." 
Physics is a body of knowledge, it is incomplete and subject to improvement (or indeed to full paradigm shifts) at all times, and the theories and principles it boasts are simply the best (or most widely accepted, rather) explanations for phenomena that someone has come up with to date.  They are certainly not absolute and therefore I don't think they are satisfactory structures to place one's understanding onto. 
I am looking for a coherent and logical way to explain this fallacy (perhaps it even has a name?),  as my knowledge of logical fallacies is limited and the few books and websites I've consulted did not help.  Thanks for your help. 

A:

There's no fallacy involved.  I'd rather say, what you deem a fallacy comes down to the practice of inferring to the best explanation.  That pattern of reasoning is sometimes said to be fallacious because it is bound to select the best hypothesis among those under consideration, which might be a bad lot after all (if you care, search for Van Fraassen's "bad lot objection").  And yet, going for the best explanation in light of the theoretical background is all you can hold a researcher accountable for, it seems to me, and it does not entail being dogmatic about it.