The Shadows are entities that claim to exist in a parallel brane (or dimension) to that occupied by humanity. In the distant past they contacted the scuttlers, who were being exterminated by the Inhibitors, and convinced them to construct a massive relay device capable of transmitting the consciousness of the shadows through branes. They provided them with mass-synthesizer technology, which the scuttlers used to consume and convert the gas giant Haldora into the transmitter -- disguising it as the gas giant once it was completed.
Before the device could be activated, however, the Nestbuilders stopped the scuttlers and obliterated them.
An indeterminate amount of time later, they contacted the human settlers of Hela -- specifically Quaiche -- in an attempt to get them to activate the mechanism that would allow them to cross over. This outcome was prevented by Scorpio, and the shadows remained trapped in their brane.
- It is heavily implied in Absolution Gap (through the description of the greenfly) that the shadows are not from a different brane, but rather from the future of this brane; humanity's future.
- There are three likely possibilities as to their identity. One, they really aren't from the future, and the similarity of the plague in their universe to greenfly is a remarkable coincidence. Two, they are humans from the distant future -- at least tens of millions of years given the distances they claim to have traveled -- who have rejected their organic bodies in favor of robotic ones as a necessity for survival. Or three, they are greenfly, pretending otherwise to travel back in time (perhaps to prevent defeat at the hands of humanity).