The least common state of matter in the universe, at least based on our current understanding, is the state known as "Bose-Einstein condensate" (BEC). A Bose-Einstein condensate is a unique phase of matter that occurs at extremely low temperatures, near absolute zero.
In a BEC, a large number of bosons (particles with integer spin, such as photons or certain atoms) occupy the lowest quantum state available to them. They "condense" into the same quantum state and exhibit macroscopic quantum phenomena. At this state, quantum effects dominate, and particles behave more like waves rather than discrete particles.
Bose-Einstein condensates were first predicted by Albert Einstein and Satyendra Nath Bose in the 1920s. However, it wasn't until 1995 that the first BEC was experimentally created using ultra-cold atoms. Since then, BECs have been observed in various systems, including dilute gases of alkali atoms.
BECs are challenging to create and maintain because they require extremely low temperatures, often achieved through techniques such as laser cooling and evaporative cooling. Given the specific conditions necessary for their formation, Bose-Einstein condensates are relatively rare in the universe compared to more commonly observed states of matter like solids, liquids, gases, and even plasmas.