I know this is kind of a philosophical question in the risk to be opinion-based, but I would like to know what the people that know a lot more than me, think about this.
I'm asking myself about this since a lot of time now. I think everything started noticing that the maximum velocity for the propagation of an "information" is the speed of light and astonished me the fact that a gravitational wave moves at $c$.
In fact eletromagnetic field is usually considered "inside" the spacetime, not part of it and it's simple to imagine a Universe with just gravity and mass; in this sense "spacetime comes first" and $c$ should be called the "gravitation velocity" or something like this.
Electromagnetic field has different properties anyway, first of all the fact that its "quantum" has no mass and no charge, while the "graviton" has mass and interacts with other gravitons.
At the same time it's strange that cannot exists an electric charge without mass, while the opposite is perfectly possible (relating to the previous "spacetime comes first"), so that, in a way, electric charge is constrained to follow the movement of the mass to which is associated and cannot detach from it. But trying to imagine it, it's very hard, it's like several dimensions are interacting one with the other, such as a zip unifies the two sides of a (multidimensional) jumper.
So I'm asking myself, is it possible that originally spacetime has something like $8$ or more dimensions, but "doesn't know where to put all of them" and for some unknown reasons, make some of them "collapsing" immersed into others; and in this case, why spacetime seems to be the master space, above the others?
Sorry if I was so qualitative, I don't really know how to express myself better. Anyway I don't want it to be only a philosophical question, but also a physical one, with a formalism I didn't put (because of my limits), but that is more than well accepted.