What would produce these sterile neutrinos? Remnants of the big bang, stars, black holes? If it is indeed dark matter and there is a lot of this in the universe, what are the theories for its formation? Can you explain a little bit about the neutrino oscillations? I'm also super confused by this figure

Are you saying the y axis is the difference in Mass and the x axis is the angle that they are being detected?

Also, I found this link that I think relates to the oscillations of neutrinos. I remember learning about eigenvalues and eigenvectors in college, but I suppose I never fully understood them

https://phys.org/news/2019-09-theorists-rosetta-stone-neutrino-physics.html