Can a theory which describes 5% of a phenomenon be considered "valid" or even useful?
If the part it describes has a practical use, yes.
(By-the-way, it's 'that' not 'which')
Our laws of physics describes the stuff we see and use in our daily lives perfectly well and has been used exclusively for the past couple hundred years. Does that not seem useful?
Actually current physics has a "practical use"... although it does not explain 95% of how Universe is made and works.
(Thanks also for grammar note. ;-) )
No, the usefulness of a theory is not in how well it describes something. Anything can be described to 98-99% accuracy, to be of any value it must be able to predict something new.
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