How do astronomers map dark matter in galaxy clusters?
Quick Answer
They look at how gravity bends light, a phenomenon called gravitational lensing, and at how fast galaxies and hot gas move within clusters. The visible matter alone is not enough to explain the observed effects.
The Full Story
Galaxy clusters are mapped using gravitational lensing, which distorts and magnifies light from background galaxies. Strong lensing can create arcs or multiple images, while weak lensing slightly stretches many background galaxies in a statistical pattern. Astronomers also measure the motion of galaxies and the temperature of hot X-ray-emitting gas, which reveals the cluster’s total mass. The difference between this total and the visible matter indicates dark matter. Famous examples include the Bullet Cluster, often cited as evidence that dark matter is separate from hot gas.
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