dwarves are from the shadowfell
In 4e cosmology, elves are the natural-world descendants of the eladrin of the Feywild. The Feywild is the bright counterpart of the Shadowfell, the land of death.
Ever since the rivalry between Legolas and Gimli, dwarves have been foils for elves. So what if dwarves descend from the Shadowfell?
It kind of makes sense. Dwarves are underground creatures who spend their time fighting the encroaching darkness. THAT’S WHAT THEY DO FOR FUN. That and build tombs and worship ancestors. And they are dour. So dour.
So let’s say that, long ago, the ancestors of the dwarves migrated from the Mountains of the Shadowfell. They established themselves in the mines and caves of the natural world, fighting goblins and kobolds instead of whatever dark shadowfell creatures they used to battle.
Does the Shadowfell still contain the Ancestors – the dwarven equivalent of eladrin? Maybe.
The Ancestors might now be enslaved to some powerful shadow creature: that long-ago flight to the natural world might have been a slave rebellion.
Or they might be taller, more graceful and more sinister than regular dwarves, with strange powers to walk through earth and stone, and with darker greeds.
They might be extinct. The story of dwarven civilization seems to be one of decay. Maybe some forgotten dwarf tunnels lead to the old Shadowfell palaces, more cunningly worked than any modern dwarven architecture, now abandoned to ghosts and balrogs and whatever else doomed the Ancestors.