ESO 325-4 (ESO 325-G 004) is an elliptical galaxy located approximately 416 million light-years away in the constellation Centaurus.
ESO 325-4 acts as a strong gravitational lens, bending the light of a background galaxy into arcs. In 2018, a combined analysis of the lensed arcs and the motions of the galaxy's own stars was used to test general relativity on galactic scales, measuring the spacetime curvature generated per unit mass as γ = 0.97 ± 0.09 — consistent with the value of exactly 1 predicted by Einstein, and at the time the most precise extragalactic test of general relativity.
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