The least common state of matter in the universe is difficult to determine with certainty because our knowledge of the universe is limited and constantly evolving. However, based on our current understanding, the state of matter known as quark-gluon plasma (QGP) is considered to be relatively rare in the universe.
Quark-gluon plasma is a state of matter that existed in the very early universe, just a few microseconds after the Big Bang. It is an extreme state where the temperatures and densities are so high that the fundamental particles called quarks and gluons, which are normally confined within protons and neutrons, are liberated and move freely. In this state, the distinction between quarks and gluons becomes blurred, and they behave collectively as a strongly interacting plasma.
Quark-gluon plasma is not typically found in our present-day universe because the conditions required to produce it are extremely rare and difficult to replicate. However, scientists have been able to create small amounts of quark-gluon plasma in laboratory experiments by colliding heavy atomic nuclei at high energies, such as in particle accelerators like the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).
While quark-gluon plasma is considered rare in the universe today, it played a crucial role in the early universe, during a phase known as the quark epoch. As the universe expanded and cooled, the quarks and gluons gradually combined to form protons and neutrons, which eventually led to the formation of atoms and the emergence of more familiar states of matter, such as gases, liquids, and solids.
It's worth noting that our understanding of the universe is constantly evolving, and new discoveries may reshape our knowledge of the prevalence and distribution of different states of matter in the cosmos.