You may also be looking for the song with the same name.The Age of Shadows was a long and dark period in the history of the Forever race.
The Age of Shadows was started by an android, TH-1, that became obsolete after the surviving Alphans ingested Liquid Eternity, and adapted to aquatic life on planet Y (Bay of Dreams, March of the Machines). In order to give his existence purpose, TH-1 started developing and expanding, and became the new 'Frame. At some point, the Alphans, having renamed themselves Forever, let go of their machine-reliant past on Alpha, which was destroyed by the old 'Frame. Due to this, they likely went along with the proposed life improvements suggested by the 'Frame, continuously making their life easier, and themselves more reliant on the machines.
Beginning sometime 6 billion years ago on the planet Y, the Age of Shadows saw the Forever reach a tipping point in their cultural and technological development where they became so reliant on machines to run their lives that they became biologically immortal but in the process lost the ability to feel emotion. This was also in conjunction with the effects of Liquid Eternity. (Age of Shadows, Liquid Eternity, The Source Will Flow) Eventually most even lost their identities, however one seemed to retain some degree of individuality. (Out of the White Hole: "Planet Y")