NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
A composite image of a spiral galaxy surrounded by a halo of neutrinos added by an artist. Neutrinos are elementary particles which were thought to have zero mass. But theoretical developments are suggesting that neutrinos with mass could solve two major cosmological questions: the unknown dark matter believed to permeate the universe and the ionisation of hydrogen clouds between galaxies. Neutrinos left over from the Big Bang, as they slowly decay into radiation, would ionise the hydrogen if they have a mass of about 30eV. And the total mass of 30ev neutrinos would account for the dark matter in the universe. The galaxy in this image is M31, the Andromeda galaxy.
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