Let's theorize that gravitational waves are ringing matter, this string theory.  Did nature or humans come up with gravitational wave sensors first?  Scientists are discovering new senses/mechanisms.  Some cultures teach their kids that you can sense when someone is talking about you, and they believe in spirits.  Like when you're falling asleep you start to hear voices but it's actually sounds from the heater or fridge or whatever.  So the gravitational waves turn into sound which turns into matter, or something like that.