The problem of explaining why the observed abundance of helium in the universe is about 25 per cent by mass. This is part of the much larger problem of accounting for the observed distribution of all the elements heavier than hydrogen. The amount of helium is much too large for it all to have been synthesized in stars, although that is where all of the heavier elements were probably made. The problem was solved by Gamow in 1946, who proposed a hot Big Bang model of the universe in which helium nuclei are manufactured at the start of the radiation era.