It is a powerful, extra-dimensional energy that can be manipulated in slightly different ways by a handful of beings that are attuned to it. There are slight yet inconclusive hints that it may be a corruptive influence of some kind (at least to Cloak, possibly to Darkhawk as well) and perhaps even a sentient entity. At least one darkforce user (Quagmire) is from the parallel universe of the Squadron Supreme instead of the main Marvel Universe. DC Comics has a similar dimension, called the Shadowlands, from which characters such as Obsidian, the Shade, and Ian Karkull draw their powers.
The Darkforce's origin is unknown. Some stories suggest that it is actually matter from a parallel universe that may be accessed by mystical means. Its first appearance was in August 1976's Champions #7, where it was shown as the source of Darkstar's powers of solid energy projection and flight.
Over the years, the Darkforce gradually became more and more of a presence among superpowered beings of the Marvel Universe. Moonstone (Karla Sofen) speculated in Avengers #238 that it could be related to the powers of the Shroud and the first Blackout. A later storyline revealed that, at the very least, it is possible to escape Blackout's energy traps by means of the Shroud's cape, as shown when Captain Marvel was trapped there by Blackout and emerged from the Shroud's cape during the Masters of Evil's attack on Avengers Mansion. New Warriors #31-33 and other storylines later expanded the number of Darkforce users even more.
Sometimes the effects of Darkforce are subtle to the point of being cosmetic. For instance, it had been deduced that both X-Men member Nightcrawler and X-Men foe Vanisher use the Darkforce Dimension during their teleportation (as revealed in Bizarre Adventures #27). However, in the Draco storyline of Uncanny X-Men, it was explained that Nightcrawler's power stems from another dimension (nicknamed the "Brimstone Dimension"). If this dimension bears any connection to Darkforce, it has yet to be revealed.