It is an odd fact about empty space that in two ways it does not seem so empty. The Bohm effect where you have a solenoid that is all wrapped up so that nearby there is not field. And yet  nearby particles definitely detect something.  There is no field there, so all there is for the particles to feel is some change in the structure of empty space  The other thing is electromagnetism. This is made by an oscillation in something.  That is what we see in the Maxwell equations. These are equations of a harmonic oscillator. But all there seems to be there is empty space. [This has nothing to do with Special Relativity which simply accounts for the fact that the Aether, even it exists is not detectable in the same way as sound waves through a medium. ]

But empty space does not seem to be the thing since we know from GR that it is hard to make a dent in it.

So where do we find this stuff? In String Theory. Open strings need something to be attached to and that is the D Branes