"Why does the universe get messier?"
Why does hot coffee always cool down? Why does a broken glass never reassemble itself? The answer lies in Entropy, a measure of disorder. The Second Law of Thermodynamics states that in an isolated system, total entropy always increases. The universe naturally flows from order to chaos.
Most physical laws are time-symmetric; they work the same forwards and backwards. Entropy is the exception. It gives time a direction—from the past (low entropy) to the future (high entropy). This is why we can remember the past but not the future.
Ludwig Boltzmann explained entropy statistically. There are infinitely more ways for a system to be messy than organized. Nature simply moves toward the most probable state. His tombstone bears the formula S = k log W.
Eventually, entropy will maximize. Energy will spread out evenly, stars will burn out, and all activity will cease. This is the Heat Death of the universe—a state of eternal, cold equilibrium.