Dark energy is the biggest mystery in modern cosmology. When observations of distinct supernova finally got good enough to observe the change in the rate of the expansion of the universe, Everyone expected to see, a deceleration due to the effect of gravity, instead astronomers discovered the universe was accelerating its expansion. Some mysterious force is acting like anti-gravity pushing galaxies (actually super clusters of galaxies) ever further about from each other. There are a number of theories explaining dark energy, very few of which are well motivated by particle physics. I have previously proposed a fifth force acting between neutrinos which I call an Axial force. I believe the Axial force offers a well motivated cause of the repulsive effect of intergalactic space.
In General Relativity both energy and pressure create gravitational pulls, and while energy is always positive, pressure can be either positive (like gas in a balloon pushing outwards), or negative when an attractive force is pulling particles towards each other. It turns out that a negative pressure causes a repulsive gravitational force. So in order to produce a negative pressure in intergalactic space, we need a substance with very little mass-energy, but a strong attractive force between its constituent particles. My (rather rough) calculations show that the attraction between the spins of neutrinos in plasma of neutrinos, is enough to cause dark energy, if the neutrinos feel the axial force (with strength 1/60 of the electromagnetic force).
In fact it give the current measured energy density of dark energy, provided the lightest neutrino has a mass of about 0.15 meV (milli-electron volts) and the next to lightest has a mass of about 8 meV. The mass difference is that measured in solar neutrino oscillation experiments, so only the lightest mass, is a prediction of the axial force theory. If this neutrino plasma is responsible for the dark energy of the universe then, we can be safe knowing that the big rip will not occur. Instead eventually the neutrino plasma will become sparse enough to decay back into a background radiation of axi-photons that will disperse away.
Dark Energy via a fifth force on neutrinos.