The helium 6 nucleus was proposed as a potential candidate to probe the neutron dark decay model. Due to the purity and great intensity of the ⁶He⁺ radioactive beam (up to 300 million nuclei per second) produced by the GANIL SPIRAL1 facility, a strict upper limit of 4×10⁻¹⁰ has been set on the branching ratio for this exotic decay mode in helium 6. This limit strongly constrains both the dark decay model and its root, the neutron lifetime discrepancy.