Physicists who feel that vacuum energy has too many problems are looking for other solutions, and one contender is a new type of particle. Physicists have observed lots of particles in the universe: the protons, neutrons, and electrons that make up atoms; the quarks that make up protons and neutrons; and particles with such exotic names as muons, leptons, and neutrinos. Dark-energy particles likely would have been created in the Big Bang and would permeate the entire universe.
Others describe a new field, like magnetic fields, electric fields, and gravitational fields, that permeates the universe, called quintessence. One key difference from the vacuum energy is that quintessence would vary with time, so it might not show up at all in the early universe but exert a powerful influence today. There are several hypotheses of quintessence fields, and in each one, each with a different outcome for the future of the universe.
So far, there’s no evidence of dark energy particles or fields. Discovering them would require not only the efforts of astronomers, but new experiments using very powerful particle accelerators.