“Nonsense! Hot air! Balderdash!” blurted out Pavel Kroupa, an astrophysicist at the University of Bonn in Germany, as I stood at the head of the lecture hall. I was just a graduate student at the time, applying for postdoctoral research positions. I had come to Bonn to give a 45-minute talk on my investigations of the small satellite galaxies surrounding the Milky Way. I had helped develop a theory that explains why these mysterious objects are located in what appears to be a straight line stretching across the sky—an unexpected and extremely puzzling alignment. Kroupa, it appeared, was not swayed by my arguments.
Most galaxies like the Milky Way are surrounded by dozens of small satellite galaxies that orbit around them. These galaxies are extremely faint—only the brightest and closest of them have been spotted flying around the Milky Way and our next-door neighbor, the Andromeda galaxy. But these dwarf satellite galaxies do not just fly around haphazardly. Instead they all sit on a thin plane, seen edge on [see box on opposite page].