The cell structure that couldn't be seen until the electron microscope was invented is the ribosome.

Here's why:

* Size: Ribosomes are incredibly small, measuring only about 20-30 nanometers in diameter. This is far smaller than the wavelength of visible light (around 400-700 nanometers).

* Light Microscopy Limitations: Light microscopes use visible light to illuminate and magnify objects. Due to the wave nature of light, objects smaller than the wavelength of light cannot be resolved. This means that ribosomes were simply too small to be seen with traditional light microscopes.

* Electron Microscopy Solution: Electron microscopes use a beam of electrons instead of light. Electrons have much shorter wavelengths than light, allowing for much higher resolution and the ability to visualize tiny structures like ribosomes.

Let me know if you have any other questions about cell structures!