The planet Su'Kal was on was composed of dilithium. Dilithium is not going to affect the entire galaxy.Neither are humans but one step across the Galactic Barrier and you get a God Complex. (Yeah you need high ESPer rating)
Su'Kal having the radiation 'connecting' him to the dilithium and it somehow sent a blast that blew up all those ships in the entire galaxy?
Around that planet? Sure. That system? Maybe, but that's a stretch. The entire galaxy?. Give me a fucking break. It was a stupid reason from writers whose work on the show was subpar, at best.