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Neutrinos, some of the most abundant particles in the universe, are also among the most mysterious. We know they have mass but not how much. We know they come in at least three types, or 'flavours' — but there may be more. A new study found that a mismatch between observations of galaxy clusters and measurements of the cosmic background radiation could be explained if neutrinos are more massive than is usually thought. It also offers tantalizing hints that a fourth type of hitherto unseen neutrino exists.

The tension between galaxy clusters and the cosmic microwave background (CMB) has been a brewing problem, albeit one that might be resolved simply by getting better measurements in the coming years (see 'Missing galaxy mass found'). The background radiation shows the small density variations in the early universe that would eventually cause matter to clump in some places and form voids in others. We can see the end product of this clumping in the recent universe by observing the spread of galaxy clusters across space.

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