Sel-Makor was created by the rift caused by Jedi and Sith interference on the planet 4,000 years before. He fed on the negative psychic energy and emotions of both sides, for four thousand years of growth. Literally all of the emotional and psychological energy of all the species on Voss fuels him. By TOR it's gotten so dangerous because the war escalates even further. Which is why he'd consume Voss and eventually the star system.
There are stated to be millions of Gormak, the Voss numbers aren't given. But both races are stated to be extremely sensitive to the Force. Then you have all of the other species. The actual population is unknown, but it's known that Sel-Makor feeds and grows on many sources of power. He was close to consuming the whole planet with his nightmare lands, prior to expanding off-world. If you take a dark-side choice as the JK, the Voss state outright that he'll become unstoppable.
He doesn't do so directly, but the Voss and the Gormak are highly Force sensitive and hate each other with extreme prejudice, Sel-Makor feeds on all of that; as well as the death that occurs. Something he did for four millennia. It only got gradually worse and by the time of the game he'd corrupt the entire planet, feeding even more to get even stronger.