Negative mass would be rather interesting wouldn't it? There's the idea that negative mass would always repel and positive mass would always attract. But that leads to the infinite acceleration problem and I believe the model beside is a better alternative. It could even solve dark matter and dark energy simultaneously, and thus there would be more negative mass in the universe. I believe it would repel light through negative curvature of space-time, but could this explain the curvature from distant objects we observe? I'm not sure. This matter would also repel leptons so they'd require their own set of them to form baryons of sorts, but I'm not sure how far the symmetry would go.