The difference between Lithium-6 and Lithium-7 might not seem like much, but it is poking holes in our understanding of how atoms formed during the Big Bang.

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Technically, they’re off by a single neutron: 6 Li contains three neutron and 7 Li contains four. Compared to theoretical calculations, experiments show that the universe contains way too much 6 Li and too little 7 Li. Using basic principles of physics, scientists can compute how much of each type of lithium should have been created during the Big Bang. But, the theory doesn’t match the data—each time they look to the sky, physicists observe a different balance of lithium than expected.

Scientists have good reason to believe their calculations are correct. They accurately predict the observed quantities of many other elements with startling accuracy.

A recent experiment published in Physical Review Letters tried to replicate the conditions of the Big Bang to prove that more lithium was produced than scientists originally calculated. The lithium would come from a fusion of a tritium atom and helium ion, which could only be replicated with a giant laser.