In Norse mythology, Svartalfheim is the realm of the Dark Elves (Svartalfar), as opposed to the Light Elves residing in Alfheim. It is sometimes confused with Nidavellir, realm of the Dwarves. Like Midgard, Jötunheim, Svartalfheim is located halfway up the cosmic tree Yggdrasil.
The distinction between Light and Dark Elves is only found in Snorri and it is likely (as Jacob Grimm first expressed it) that it is the transposition of the Christian dualism angels/demons.
However, it is possible that it refers to two aspects of the cult of the Elves: the cult of fecundity for the luminous Elves, the cult of the dead for the dark Elves.