The least common state of matter in the universe is difficult to determine with certainty, as it depends on various factors such as temperature, pressure, and cosmic context. However, one state of matter that is relatively rare in the universe is the so-called "degenerate matter."
Degenerate matter is a state of matter that occurs under extreme conditions of high density and low temperature. It is typically found in stellar remnants such as white dwarfs, neutron stars, and possibly in the cores of massive stars during supernova explosions.
In a white dwarf, for example, the matter is so densely packed that the electrons become highly compressed and occupy a range of energy levels. This compression gives rise to a degenerate electron gas, where the electrons are no longer free to move and behave as they do in ordinary matter. Instead, they obey the principles of quantum mechanics, filling up available energy states from the lowest upward, resulting in a high-pressure, electron-degenerate state.
Neutron stars are another example of degenerate matter. In these extremely dense objects, the pressure is so intense that the electrons and protons combine to form neutrons. The core of a neutron star consists primarily of a degenerate neutron gas, where the neutrons are densely packed and subject to quantum mechanical effects.
Considering the rarity of white dwarfs and neutron stars compared to other forms of matter, it can be argued that degenerate matter, in its various manifestations, is among the least common states of matter in the universe.