The expansion of the Universe describes how space itself is stretching, causing galaxies to move away from each other over time. Unlike objects moving through space, galaxies are carried along by the expansion of space itself—imagine dots on a balloon's surface separating as the balloon inflates.</p><p>This phenomenon was first observed by Edwin Hubble in 1929, who discovered that distant galaxies appear redshifted (their light stretched to longer wavelengths) and that more distant galaxies recede faster. This relationship, known as Hubble's Law, shows that a galaxy twice as far away moves away twice as fast, with the current expansion rate (Hubble constant) measured at approximately 70 kilometers per second per megaparsec.</p><p>The expansion explains several key observations: why the night sky isn't uniformly bright despite containing billions of stars (Olbers' paradox), the cosmic microwave background radiation from the Big Bang, and the observed abundances of light elements like hydrogen and helium. Remarkably, General Relativity predicted this expansion before it was observed—Einstein initially added his "cosmological constant" to prevent it, later calling this his "greatest blunder."</p><p>Today, we know the expansion is actually accelerating due to mysterious dark energy, comprising about 68% of the Universe. This discovery earned the 2011 Nobel Prize in Physics and fundamentally changed our understanding of cosmic destiny.