The Void is a region of space that the USS Voyager encountered on Stardate 52081.2, (See VGR "Night"). It is a region containing no stars and a form of radiation that prevents the stars outside the region from being seen.
When Voyager encountered the Void, the crew was facing a daunting two year journey through the region. This had an adverse effect on the moral of the crew and their captain. However, this trip was cut short when the crew encountered a Malon Garbage Scow which showed them a shortcut to the other side of the region.