Right now, telescopes can see distant galaxies violently colliding at millions of miles per hour. Yet if you watched one for an entire lifetime, you wouldn't see a single pixel move.
Galaxies are unfathomably large. The Milky Way is about 100,000 light-years across, and the distances between galaxies are measured in millions of light-years. Because of this immense scale, a single collision takes hundreds of millions to billions of years to complete. To human eyes, these high-speed collisions appear completely static.
Because astronomers cannot watch a single collision unfold, they study galactic mergers using a cosmic flipbook. By surveying the sky, they find different galaxies caught in varying stages of the process. They observe some pairs just beginning to approach one another, others like the Antennae Galaxies (pictured above) whose gravitational interactions have violently deformed their structures into long stellar "tails", and finally, older systems that have fully merged into a single elliptical galaxy. By lining up these frozen snapshots, astronomers piece together the complete sequence of a collision.
When these structures merge, it is a gravitational dance rather than a physical crash. The spaces between individual stars are so vast that actual stellar collisions are incredibly rare. Instead, the real impact occurs when massive clouds of interstellar gas slam together. The resulting compression triggers fierce bursts of star formation, lighting up the merging galaxies with dense clusters of young, brilliant stars.
The Milky Way is currently on its own collision course with its nearest massive neighbor, the Andromeda Galaxy. Andromeda is rushing toward the Milky Way at 250,000 miles per hour, but because of the distances involved, the two will not meet for another 4.5 billion years.