Andromeda Galaxy is the most distant naked-eye object visible in the sky and it has been a subject of astronomical wonder from ancient history. This talk examines the history of the astronomical research through the study of Andromeda Galaxy. It discusses how M31 was described both among the Ancients, but more importantly, among astronomers from the nineteenth century to the present. While at the start of the twentieth century the universe was depicted as a finite cosmos dominated by the Milky Way, the study of Andromeda Galaxy shattered that image, creating an infinite universe of countless galaxies and vast distances. Even today, M31 is a major focal point for new astronomical discoveries, and it also remains a popular celestial object among amateur astronomers to observe and study. The conclusion is that there is a hidden history of M31, one that reveals it to be an important astronomical object central to astrophysics, critical to gathering knowledge about the reality of the universe, and as a object of folklore and interest among amateurs.