Neutrinos were first produced in the universe some 14 billion years ago, 10 to the -43 seconds after the Big Bang. A mere second later, they were already rapidly moving away from the rest of the hot and dense primary particle soup; scientists are still seeking to detect these neutrinos that survive from the Big Bang. So far, only two sources of extraterrestrial neutrinos have been observed: the sun and supernovae.
The number of neutrinos emitted by the sun that reach us on the Earth is measured by the “flux”. The solar neutrino flux for us on Earth is about 65 billion neutrinos passing through just one square centimetre of area on Earth, every second. That’s a lot of neutrinos! And almost all of them pass right through the Earth and out the other side (but more on that “almost” later). That means that every second, trillions of neutrinos are passing through your body, since you too are transparent to them.
Though we don't have to look as far as outer space to find neutrinos. On Earth, both natural and artificial neutrino sources exist: radioactive material from inside the Earth's core can undergo beta decay, producing geo-neutrinos. Additionally, neutrinos are a byproduct of the fission reactions in nuclear reactors. And finally, to study the properties of these special particles, dedicated particle accelerators are used to produce neutrinos which can then be detected in a neutrino detector.