An astronomer is examining quasars and happens upon two quasars in nearly the same part of the sky that have identical spectra. They cannot see any massive object in the region between the two quasars. What is a reasonable explanation for this discovery?

Question

Quizplus · Accepted Answer

Gravitational lensing can create multiple images of the same astronomical object, such as a quasar, when a massive object (like a galaxy or a cluster of galaxies, often composed largely of dark matter) bends the light from the quasar around it. The absence of a visible lensing object does not rule out gravitational lensing, as the lensing mass can be primarily dark matter, which does not emit light and is therefore not directly observable.

An Astronomer Is Examining Quasars and Happens Upon Two Quasars