Take a look out at almost any galaxy in the Universe, and you’ll find it’s moving away from us. The farther away it is, the faster it appears to recede. As light travels through the Universe, it gets shifted to longer and redder wavelengths, as though the fabric of space itself is being stretched. At the largest distances, galaxies are being pushed away so rapidly by this expanding Universe that no signals we can possibly send will ever reach them, even at the speed of light.
But even though the fabric of space is expanding throughout the Universe — everywhere and in all directions — we aren’t. Our atoms remain the same size. So do the planets, moons, and stars, as well as the distances separating them. Even the galaxies in our Local Group aren’t expanding away from one another; they’re gravitating towards one another instead. Here’s the key to understanding what is (and isn’t) expanding in our expanding Universe.