gargantuan Bang neutrinos are believed to be everywhere in the universe but have never been seen. The expansion of the universe has stretched them and they are thought to be billions of times colder than neutrinos that stream from the sun. As the oldest known witnesses or 'relics' of the early universe, they could shed modern light on the birth of the cosmos if scientists could pin them down. That's a tall order since these ghostly particles can speed through planets as if they were empty space.
Source: phys.org