String theory and its extensions aim at unifying the four known interactions in one mathematical model based on quantum mechanics. The first three are already almost there in the GUTS theories, and the holy grail is for gravity to join them.
String theories will not change the standard model, or the GUTS fits, but will embed them in a mathematically satisfactory way, that is the aim. It may explain discrepancies with the standard model , as CP violation or flavor violation or similar effects, but nothing drastic.
Dark matter and dark energy come from astrophysical observation and are necessary to fit the Big Bang cosmological model. Dark matter in the model is based on Newtonian mechanics. Dark energy needs general relativity equations. It is expected that general relativity will emerge naturally in the final model of string derived theories. That is the aim.
So the explanation will be the same as today, when there is no final string theory model, because the existing theories , standard model, Guts, and General relativity will not change once there is unification of the four forces. There may be new predictions.
When a definitive string/M model is proposed, it is possible that new experimental observations might be predicted, due to the extra dimensions of string theories, but this will be a research project for the future. For dark matter the candidates will still be as seen in this article, and dark energy will also be dependent on the constants entering general relativity equations. Unification of all four forces will not solve all astrophysical problems.
comment by OP :
Why even bother trying to unify incomplete theories. And how can you even begin to think that you will make a fundamental unification and have no gain out of it( predictive or otherwise)
If Maxwell had not bothered to unify electricity and magnetism, would we have this technical civilization? Electricity and magnetism were incomplete theories then. At that time it was a brilliant insight.
The theorists working on string type theories aim on unifying all four forces are following in those steps. History of science up to now tells us that new theories give rise to new possibilities in technology. It is a gamble theorists are willing to take, and the physics community in general. Doing research is a gamble.