Neutrinos have mass... insignificant mass, we are told, as their speed is so near that of light we can't even observe the difference. On the other hand, there are a lot of them. Every year the Sun produces something like half a tredecillion neutrinos, if my math is right. Depending on the mass of a neutrino (somewhere between 0.005 and 0.05 eV) this could be anywhere between 5 and 50 metric tonnes. Someone please check my calculations because that seems like an awful lot of mass. Maybe it's not anything in comparison the entire mass of the Sun, but remember that there are 70 sextillion stars in the observable universe, which is billions of years old. That's at least a supermassive black hole in terms of the mass of neutrinos that have been produced. Considering how little they interact with matter and how vacuous space is, I would assume they're pretty much all still out there. Where do they go? Are they still pushing on the boundaries of space? Have they been slowed in their progress by the gravitational pull of the universe, maybe even orbiting massive structures like superclusters? Or have they in fact mostly annihilated each other or been absorbed into neutron stars and black holes?