Gravitational merger — the macro twin of D+D fusion. Two concentrations of positive mass on the x-axis attract by self-gravity and merge into one — the same Poisson-coupled continuum mechanics that fuses two deuterons by Coulomb, here at cosmic scale.   Δφ + Gρ = 0  |  ρ_t + div m = 0  |  m_t + div(m v) + grad p − νΔv = ρ gradφ  |  e_t + div(e v) + p div v − φ div m = 0;   p = γe, v = m/|ρ|.   Same-sign masses attract (gravity); opposite signs would repel (dark energy). Red = positive mass; white dots = fluid tracers; periodic box.