Part 6/9:
A persistent criticism of string theory is its "where's the beef?"—the lack of direct evidence for predicted particles. String theory predicts not only the particles we observe but also heavier, unseen particles. These could potentially make up dark matter, the mysterious, invisible substance that constitutes about 27% of the universe's mass-energy content. Astronomers observe dark matter's gravitational effects on galaxies, but it remains undetectable by traditional means.