Axion

Axions have no electric charge, a very small mass, and very low interaction cross-sections for strong and weak forces. Hence they are nearly invisible to ordinary matter, and are incredibly difficult to observe.

Axions were created abundantly during the big bang. Because of a unique coupling to the instanton field of the primordial universe, an effective dynamical friction was created during the acquisition of mass following cosmic inflation. This robbed all such primordial axions of their kinetic energy. Hence the Universe is filled with a very cold boson condensate of primordial axions.

The superpartner of the axion is called the axino.