Which realm of fire existed before the world?

The Norse realm of fire that existed before the world was Muspelheim, also called Muspell.

In the creation account of Gylfaginning, Muspell is a blazing southern region filled with heat, flames, and sparks. It existed before the earth was formed, alongside the icy, mist-filled realm of Niflheim. The boundary between these opposing forces was Ginnungagap, the primordial void.

Heat and sparks from Muspelheim met frost and poisonous ice flowing from Niflheim. The resulting melting and dripping produced Ymir, the first primordial being, while sparks eventually became the sun, moon, and stars. Muspelheim therefore plays an active role in creation, not merely a background role as a place of fire.

Muspelheim is commonly confused with Niflheim, which represents cold, mist, and ice, or with Asgard, the stronghold of the gods. It is also associated with Surtr, a fire giant or fire-being who guards the realm and leads the forces of fire during Ragnarök. The spelling Muspell reflects the older name found in the myths.