The Milky Way is 'rippling' like a pond, and scientists may finally know why

Imagine the Milky Way's 100 billion stars as a flat, tranquil pool of water. Now, picture someone dropping a stone the size of 400 million suns into that water. The tranquility is shattered. Wave after wave of energy ripples across the galaxy's surface, jostling and bouncing its stars in a chaotic dance that takes eons to calm.
Astronomers suspect that something like this may have really happened — not just once, but several times over the past several billion years.