"Cataclysmic crashes involving black holes and ultradense neutron stars may explain the briefest of the most powerful explosions in the universe, scientists say."
http://news.yahoo.com/kilonova-dead-star-crashes-may-spark-mysterious-cosmic-212239960.html
Output of the clash of neutron stars and black holes is energy and rare/valuable metals.
That's an opposing forces result if I've ever seen one.