Why have the extra dimensions proposed by string theory not been observed?

In string theory, the extra spatial dimensions have not been observed because they are believed to be "compactified"—or crumpled—to such an infinitesimal size that they remain beyond our current ability to detect them.12

Physicists explain this using the analogy of a garden hose: from a distance, a long hose appears to be a one-dimensional line. However, upon closer inspection, it possesses a circular cross-section that represents an additional dimension. Similarly, while the three dimensions of our common experience are large and manifest, the six extra dimensions required by string theory are thought to be curled up so tightly that they evade observation by the naked eye and current experimental instruments.1

Because strings are so small, they vibrate within these hidden dimensions. The specific geometric form of these dimensions influences how strings vibrate, which in turn determines the fundamental properties of particles, such as their mass and charge. A significant challenge for researchers is that these dimensions can take many different geometric shapes, making it difficult to extract definitive, testable predictions from the theory.1