= Antimatter
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Antimatter consists of <antiparticles>, whose <rest masses> and <spins> match those of the corresponding <particles> while additive internal <conserved charges> are reversed. A <Positron>, for example, has the <electron>'s <rest mass> and opposite <electric charge>. Both have positive physical <energy>.
When a <particle> meets its <antiparticle>, <annihilation> can convert the pair into other <particles> while preserving total <energy>, <momentum> and applicable <conserved charges>. <Electron-positron annihilation> into two <photons> is a simple example. Producing a few <antiparticles> in energetic collisions does not establish a large cosmological antimatter population: the production process and the subsequent abundance are separate questions. The <antiparticle> description is built into relativistic <quantum field theory>, rather than requiring negative measured <mass> or <energy>.
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