Sometime in the far distant future, about 4 billion years from now, our Milky Way galaxy will collide with the neighboring Andromeda galaxy, as illustrated in this artist’s rendition.
The universe is expanding and accelerating, and collisions between galaxies in close proximity to each other still happen because they are bound by the gravity of the dark matter surrounding them. The Hubble Space Telescope’s deep views of the universe show such encounters between galaxies were more common in the past when the universe was smaller.
A century ago astronomers did not realize that M31 (also known as Messier 31 or the Andromeda Galaxy) was a separate galaxy far beyond the stars of the Milky Way. Edwin Hubble measured its vast distance by uncovering a variable star that served as a “milepost marker.”
Hubble went on to discover the expanding universe where galaxies are rushing away from us, but it has long been known that M31 is moving toward the Milky Way at about 250,000 miles (400,000 kilometers) per hour. That is fast enough to travel from here to the moon in one hour.