When we look out at distant galaxies, we find that almost all of them are moving away from us. But this isn't because we are at the center of some cosmic explosion. Instead, the very stretching of space itself is pushing the galaxies apart, like dots drawn on the surface of an inflating balloon.

To map this cosmic growth, astronomers rely on a phenomenon called redshift. As a light wave travels from a distant galaxy toward Earth, the expanding space it passes through stretches the wave, shifting its color toward the red, longer-wavelength end of the spectrum.