Second Law of Thermodynamics, or, Einstein is a babe.

Heat cannot be transferred from a colder to a hotter body. As a result, natural processes that involve energy transfer have one direction, thus all natural processes are irreversible. This law predicts that the entropy of an isolated system always increases with time. Entropy is the measure of the disorder (or, randomness) of energy and matter in a system. This may mean that both energy and matter are becoming less useful as time goes on.
And here is the reason we are so obsessed with working out what the universe looked like in the microseconds after the big bang; energy and matter were unified and perfect order occurred. The rapid expansion of the universe and its continued expansion today, the cooling from its dense hot state to our more temperate climate, dark matter and dark energy are all linked back to this second law, forever taking a backseat to e=mc2.