Is There a Fourth Neutrino Out There in the Universe?

Meanwhile, we knew from particle colliders that there could only be three types of neutrino that coupled to the Standard Model particles, and we learned mass limits on the sum of neutrinos from cosmological observations. (SIMON SWORDY (U. CHICAGO), NASA)From all of this, we were able to conclude:
there are three types of neutrino,
they have tiny, non-zero masses,
they oscillate over large distances from one flavor (electron, muon, or tau) into another,
and they can only make up a tiny fraction of the dark matter. In the physical models we make, there are simple relationships between the distance a neutrino travels, the neutrino energy, and the differences in mass between the different types of neutrinos.

Source: medium.com