Yale Astronomy Public Thesis Talk - Michael Keim

The night sky is filled with communities of stars known as galaxies. Until recently, it was thought that an even greater amount of unseen mass known as “dark matter” dominated all of these galaxies and governed the motions of stars within them. However, recently a pair of unusual ‘ghostly’ galaxies known as DF2 and DF4 have been found to have no dark matter at all. Here we delve into these unusual objects, thought to have formed as part of a high-speed collision, culminating in the discovery of a third galaxy without dark matter: DF9. These findings have reshaped standard models of galaxy formation, and provide laboratories to explore the fundamental nature of dark matter.