Compressible flow + gravitation, mass of variable sign.   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|.   Two galaxies of opposite-sign mass repel (dark energy); same sign attract. Red = positive mass, blue = negative mass; white dots = fluid tracers; periodic box.