"The Void" was the name given by the crew of USS Voyager to a vast region in the Delta Quadrant where no star systems existed within 2,500 light years, and where all light from beyond was occluded from sensors by heavy concentrations of theta radiation.
The Void was inhabited by at least one species, whose physiology had evolved to survive in complete darkness. In addition, a spatial vortex existed within the Void, which led to an area just outside the region.
When Voyager entered the Void in 2375, they found themselves faced with a daunting two-year journey to cross it, expecting to leave it in 2377. The oppressive nothingness had an adverse effect on the morale of the crew, including Captain Janeway, who secluded herself in her quarters, obsessing over her decision four years previously which stranded Voyager in the Delta Quadrant. The crew also found themselves with an abnormal amount of free time on their hands, leading to high demand for the holodecks, and inspired Ensign Kim to compose a concerto on his clarinet titled, "Echoes of the Void."
After two months, the crew discovered that the Malon were using the vortex to dispose of antimatter waste, poisoning the indigenous species with theta radiation. To protect the aliens from the Malon, Captain Janeway conceived of a plan whereby a spread of photon torpedoes would be used to destroy the vortex. However, since the vortex could only be collapsed at the end which existed in the Void, where its dimensional radius was weakest, doing so would preclude the possibility of using the vortex to travel to the other side. Consequently, the crew modified their plan, deploying the torpedoes immediately upon entering the vortex. Reinforcing their aft shields to withstand the shock wave, the crew was able to safely navigate their way through the vortex, exiting beyond the boundary of the Void. (VOY: "Night")
In the String Theory trilogy, it is revealed that the Void was caused by Voyager helping to seal a rift in space that opened up to Exosia, the home dimension of the Nacene. As the rift was sealed, the space that it occupied in space/time changed to accommodate thousands of years of another history, and the absence of light is due to the photonic energy in that area having been drained into the rift and unable to regenerate at this time.