Hmmm. Does anti-matter have negative mass? If it does, and if a mirror universe were made of it, then the Big Bang could have had conservation of mass. I don’t see how “negative mass” is possible, though. Course I grew up in this universe learning the rules that apply here, so.....
Negative mass would depend on which way one’s scales measured, wouldn’t it? The other universe would likely believe we were measuring negative mass.
(And I might be skinny in the alternate universe!)
Antimatter has opposite charge but normal mass. Matter with truly negative mass is hypothetically called “exotic matter”, and would be a necessary component of things like wormholes, warp drive, etc. And if it interacts with normal matter, instead of exploding as pure electromagnetic energy it would just cancel out.