My pet hypothesis is that it's all virtual particles. Sometimes stuff just happens. Literally, stuff happens. Matter spontaneously emerges from a vacuum, as a consequence of quantum fluctuations. A divot and a hole form at the same time. When they rejoin, they zero out. And they will tend to rejoin immediately because they both have mass and there's nothing else for a zillion miles.

We observe this as the Casimir effect. Stick two flat surfaces really close together, in total vacuum, and they can act like there's gas pushing them apart. The divot and hole bounce off separate things. Clever tricks to abuse this have not worked: see every EM engine blasting energy into a vacuum chamber. As an engineer, not a scientist, I suspect some of the particles being blasted get negative momentum, somehow, and that's why the net force is immeasurable. Virtual particles play by their own rules.

But virtual particles still have mass. They bend spacetime as gravity. Cancelling that out would require negative mass, and antigravity, which sadly seems too cool to exist. This means all those pseudo-atoms in deep space drag things toward them. It looks like there's stuff out there, because... there was. Briefly.

This could also explain universal expansion. This transient noise in the fabric of reality means empty space has mass. So whatever we consider our universe, it's surrounded on all sides by unfathomable quantities of nothing, and the nothing is tugging it in all directions. It's not just escape velocity from the initial explosion; things are accelerating.