A mysterious cosmic explosion created a brilliant flash of light in the space between two galaxies over 3 billion light-years away. The optical flash, which was one of the brightest bursts of blue light in the universe but lasted only a few days, is the latest example of a rare breed of brief astronomical event called a luminous fast blue optical transient (LFBOT). LFBOTs are a complete mystery. The first one to be discovered wasn't observed until 2018. Designated AT2018cow, it was positioned in the spiral arm of its galaxy 200 million light-years away. Nicknamed "the Cow," it was up to 100 times brighter than an ordinary supernova, and was also bright in radio waves, ultraviolet and X-rays. If it was a supernova, it behaved very oddly. Usually, a supernova stays bright for weeks, or even months, and has a recognizable spectrum. Yet the Cow faded after a few days.