  We show that a single dark soliton can exist in a spin-orbit-coupled Fermi
gas with a high spin imbalance, where spin-orbit coupling favors uniform
superfluids over non-uniform Fulde-Ferrell-Larkin-Ovchinnikov states, leading
to dark soliton excitations in highly imbalanced gases. Above a critical spin
imbalance, two topological Majorana fermions (MFs) without interactions can
coexist inside a dark soliton, paving a way for manipulating MFs through
controlling solitons. At the topological transition point, the atom density
contrast across the soliton suddenly vanishes, suggesting a signature for
identifying topological solitons.
