Simulation: A Disk Galaxy Forms . ... Green depicts (mostly) hydrogen gas in the above movie , while time is shown in billions of years since the Big Bang on the lower right. ... Soon numerous proto-galaxies form, spin, and begin to merge . ... For our Milky Way Galaxy , however, big mergers may not be over -- recent evidence indicates that our large spiral disk Galaxy will collide and coalesce with the slightly larger Andromeda spiral disk galaxy in the next few billion years. ...