The Deuterium Bottleneck

The mass-2 isotope of hydrogen, deuterium, plays a pivotal role in the early big bang for two reasons: it is the simplest nucleus that can be made beyond hydrogen, and it has a very low binding energy so that it breaks apart easily in a collision.

Because of these properties, it serves as a bottleneck to the synthesis of nuclei heavier than hydrogen. They cannot be made until deuterium can hold together to serve as a seed. This isn't possible until the temperature drops low enough that deuterium is not likely to be torn apart by collisions in the gas.