And that extra weight could make a big difference in a billion years or so if the universe, currently expanding from the Big Bang, suddenly falls back in on itself. "Add in the extra weight of the neutrinos, and the total mass of the universe is near that critical point," says TIME science correspondent Mike Lemonick. "This a big discovery."
In the short term, the success of the Japanese experiment could change a few of physics' fundamental laws -- if the results hold up. "The concept of the massive neutrino has been proven -- and then disproven -- before," adds Lemonick. "But these guys are considered the best in the world, and their facility is considered the best in the world. And they're confident. So this is very credible."