Abstract <p>The existence of the dineutron was predicted 75 years ago. At present, a number of experimental works confirm this assumption. By virtue of the principle of isotopic invariance, a singlet deuteron must also exist. The possibility of describing the neutron-proton interaction in the state at low energies as the excitation of a quasi-stationary level lying below the deuteron (singlet deuteron) decay threshold is discussed. The position, neutron and radiative widths of the level are determined by the scattering length, the effective radius, and the cross section for the radiative capture of neutrons by protons. Experiments to search for this level are discussed. In accordance to the principle of isotopic invariance the discovery of the singlet deuteron would be confirmation of the existence of the dineutron.</p>