When did vacuum energy of the Universe become cosmological constant?
  A quark-gluon phase transition in the Universe is researched after which vacuum (dark) energy has hardened and become cosmological constant. Before this a vacuum component of the Universe was changing by jumps during phase transitions since vacuum condensates of quantum fields carried a negative contribution in its positive density energy. This quintessence period of the Universe life took place during the first parts of a second when our Universe was losing high symmetry. Using Zel'dovich's formula the modern value of vacuum energy is also calculated. It is shown that a quantum chromodynamical vacuum which is characterized by pseudogoldstone bosons existed definitely when temperature of the Universe was T~150 MeV. Therefore there is a large probability that dark energy is vacuum energy. 
V.Burdyuzha