Almost three years ago, astronomers using the Hubble Space Telescope were perusing a cluster of galaxies about eight billion light-years from Earth when they came upon a flash of light unlike anything they had seen before.
Over the next 100 days, the object gradually brightened. Then it spent another 100 days growing dimmer, until it finally vanished from view.
Astronomers speaking last week at the American Astronomical Society meeting in Long Beach, Calif., still have no idea what it was — or is...