The Milky Way is made up of thousands of "substructures such as streams." These "streams" are really clumps of stars – moving at a shared velocity, in a shared direction – that were "ripped" from other galaxies, "most likely small galaxies that were pulled in to create the Milky Way." Streams at the center of the galaxy are thus the oldest, as "you probably build a galaxy up from the inside out." Indeed, the sky is criss-crossed with rivers that flow from the remains of structures unspooling elsewhere.