The nongravitational interactions of dark matter in colliding galaxy clusters

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Science  27 Mar 2015:
Vol. 347, Issue 6229, pp. 1462-1465
DOI: 10.1126/science.1261381

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Uncloaking the influence of the invisible actor

The idea of dark matter enjoys popular support, but two major concerns persist: the so-called Standard Model excludes it, and it cannot be directly detected by any telescope. For now, astronomers can only observe dark matter's influence indirectly, such as when watching unseen creatures perturb the surface of a pond. Harvey et al. observed 72 galaxy collisions to compare the resulting centers of mass for the gas and stars (from direct observations) and for the dark matter (by inference). Based on these offsets, dark matter is clearly present.

Science, this issue p. 1462


Collisions between galaxy clusters provide a test of the nongravitational forces acting on dark matter. Dark matter’s lack of deceleration in the “bullet cluster” collision constrained its self-interaction cross section σDM/m < 1.25 square centimeters per gram (cm2/g) [68% confidence limit (CL)] (σDM, self-interaction cross section; m, unit mass of dark matter) for long-ranged forces. Using the Chandra and Hubble Space Telescopes, we have now observed 72 collisions, including both major and minor mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6σ significance. The position of the dark mass has remained closely aligned within 5.8 ± 8.2 kiloparsecs of associated stars, implying a self-interaction cross section σDM/m < 0.47 cm2/g (95% CL) and disfavoring some proposed extensions to the standard model.

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