Complex sentence: Most quasars were formed approximately 12 billion years ago caused by collisions of galaxies and their central black holes merging to form either a supermassive black hole or a binary black hole system.
Split into simpler sentences: Most quasars were formed approximately 12 billion years ago. They are normally caused by collisions of galaxies, with the galaxies' central black holes merging to form either a supermassive black hole or a binary black hole system.