The reason the universe is not at zero entropy is a topic of ongoing scientific investigation and debate. Entropy is a measure of the disorder or randomness in a system, and the second law of thermodynamics states that the entropy of a closed system tends to increase over time.
The universe, as a whole, is often considered as a closed system for the purpose of discussing entropy. However, it's important to note that the universe's overall entropy is not fully understood, and there are various theories and hypotheses attempting to explain its current state.
One hypothesis is that the universe started in a state of extremely low entropy during the Big Bang. This initial low entropy state is sometimes referred to as the "cosmic egg" or "primal state." As the universe evolved and expanded, entropy gradually increased. This increase in entropy is associated with processes such as the formation of stars, galaxies, and the eventual dispersion of matter and energy throughout the universe.
If the universe were to reach a state of maximum entropy or zero usable energy, it would be in a state known as the heat death or the maximum entropy state. In this state, all energy would be uniformly distributed, and there would be no gradients or areas of lower entropy available for energy transfer or useful work. Essentially, all processes would cease, and the universe would be in a state of equilibrium.
However, it's important to note that the concept of the heat death of the universe is based on current scientific understanding and theories. There are ongoing debates and investigations into the ultimate fate of the universe, and it is still an active area of research. New discoveries and developments may lead to revised or refined understandings of entropy and the future of the universe.