The largest fraction of matter in our Galaxy is invisible. It might consist of particles similar to neutrinos but much more massive, the ‘neutralinos’. Let us indicate them by χ and let m_χ be their mass. According to one theory these particles coincide with their antiparticles. The annihilation processes χ + χ → γ + γ and χ + χ → Z⁰ + γ would then take place. Assume initial kinetic energies to be negligible. If a gamma telescope observes a monochromatic signal E_γ = 136 GeV, find m_χ in both hypotheses.