... Tonight's Sky . ... Observing . ... Gravitational wave search provides insights into galaxy evolution and mergers . ... Dark-matter dilemma . ... Optical telescopes can observe the starlight from the individual galaxies, and can infer the location of dark matter by its subtle light-bending effects on distant galaxies. ... A popular theory of dark matter predicts that dark matter and galaxies should stay together, even during a violent collision, as observed in the case of the so-called Bullet...