Space centaurs are giant comets up to 62 miles across. They tend to hang out in the Kuiper belt region. This means they're not an immediate danger by default but can be bumped into the inner solar system when sufficiently large objects pass by. Objects like ... the theorized ninth planet. Crap.
"Yeah, bet you wish you'd stuck with me now, don'tcha?"
In the unlikely event that Bruce Willis fails to stop it, a centaur getting too close to Earth would wreak havoc. Its trip from outer to inner space and through our atmosphere would likely reduce its size significantly, but the disintegration would loose enough dust to reduce our sunlight to the level of moonlight. Of course, the dust would eventually clear up ... after 100,000 years. And that's assuming that what remains of the centaur doesn't then impact Earth and level humanity. Yes, "perpetual Goth phase" is the best possible outcome of that particular disaster scenario.