[SolarNet]

He points out that it's a mathematical tool and may not result in particles, e.g. matter or energy, as we currently think of it.

It could be a property of space-time, as space is always expanding everywhere, space it self could have a negative mass.

This could even be some sort of weird energy-neutral balancing mechanism (e.g. some sort of energy - my bet is on potential energy, like how gravity causes things to accelerate through the potential energy of the arrangement - is lost that accounts for the creation of negative mass particles or whatever; conservation is observational, but we don't really know what 95% of the stuff being conserved is in the first place).

Hence it can be consistent with the conservation of mass and energy.