The mysterious destruction of most dilithium in the galaxy.
The Burn was a disaster in 3069, when almost all the dilithium in the galaxy suddenly went inert. Ships using dilithium for warp travel exploded in an instant, killing huge numbers of people, and the Federation, which relied on warp travel to hold together, lost much of its membership.
Many worlds withdrew, including Earth and Ni'Var, and the Federation was reduced to a small, hidden headquarters under Admiral Charles Vance. With dilithium scarce, groups such as the Emerald Chain, led by Osyraa, took control of trade and much of the galaxy.
When the USS Discovery arrived in 3188, Michael Burnham and her crew traced the Burn to the Verubin Nebula. There they found Su'Kal, a Kelpien who had been born on a crashed research ship and had grown up alone on a planet rich in dilithium. When he was a child, his cry of grief at his mother's death had caused the Burn. Saru helped him, and Discovery's discovery of the planet's dilithium helped restore travel and rebuild the Federation.