The Void was a region of space in the Delta Quadrant, perhaps 9 light-years across, which was shielded off from the rest of the galaxy by a subspace barrier. The void was (seemingly) entirely empty. Matter was sucked into the void periodically by vortexes which opened and closed sporadically. Many spacecraft were engulfed in such a way, and were fored to live in the void, having no way to escape. All the inhabitants of the Void became accustomed to preying on each other-- and on new arrivals to the Void-- to survive.