High energy physics and the standard model have unequestionable come up with all kind of different particles, which are unlike the matter of which our own world exists (well they exist of course in our own world, but ordinary atoms is made of only some of those particles). As for instance the concept of anti-matter, which are existing forms of matter, but are very rare (since they annihilate with normal matter). However, all particles are either having a zero mass or a positive mass. In which way are particles with negative mass "forbidden" by the standard model and the known laws of physics? How would such particles behave, if they could exist? (what if similar particles, but one with positive mass and other with negative mass would annihilate? It would not leave any energy, unlike matter - anti-matter annihilation). (I assume for instance the negative matter - i.e. matter with a negative mass - are in many ways the same, also excert gravitational force, but only in the interaction with normal (positive mass) matter, they would behave repulsive.) I assume that if negative matter could exist, we would sure detect it. But what "forbids" nature to have negative mass particles?