String theory literature tells us that ST predicts the four forces: weak, strong, EM, and gravity. What it fails to tell us is if that's all the forces it predicts. Might there be a fifth force that ST predicts, but was thrown out because there is no observable evidence in 4D space that a fifth force exits?
Might that fifth force, if there is one, account for the so-called dark energy in the universe?
Greene, Witten, Kaku, and others on the string train all mention the "four forces" unified by ST. But none mentions if only those four forces are predicted by ST. Might it be possible the fifth and higher forces predicted by ST are simply not mentioned because there is no evidence in the standard model of a fifth or higher forces.