In 1991, the Hubble Space Telescope photographed the Andromeda galaxy and revealed that a double nucleus lies at its center. A nucleus is a dense group of stars at the midpoint of a galaxy. The most likely explanation for this discovery is that at some point Andromeda absorbed a smaller nearby galaxy, the nucleus being the only remaining part of that galaxy. Another possibility is that a lane of interstellar dust runs through the middle of one wide nucleus, giving it the appearance of two separate nuclei.
