After Einstein finished his theory of general  relativity, he wanted to explain where matter  

came from; he was looking for a unified theory.  His main idea seems to have been that matter  

is really made of space-time. We presently  assume that matter, or other types of energy  

cause space-time to curve. And the effect  of that curvature on matter is what we call  

gravity. But Einstein was looking for a way that  space-time would curve on its own and that curved  

spacetime would then be what we call matter  today. Unfortunately, he died before he found  

a way to make it work. I quite like the  idea and wish someone would pursue it.