Axion

The axion is a hypothetical particle devised by theoreticians to explain the charge-parity symmetry observed in the strong interaction.

The axion could be the invisible matter of the universe.

This particle is very light (its mass should be between 10^-6 and 10^-3 eV/c²), is electrically neutral and only interacts very weakly with "ordinary" matter. It is therefore very difficult to detect.