To cosmologists it means " nonrelativistic particles ", which have the property that they dilute by a factor of x 3 when the universe expands by a linear factor of x ( or a volume factor of x 3 ) . " Radiation " is relativistic particles, which dilute by a factor of x 4 instead.
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Between the straws, materials with widely varying relativistic particles ( those with a speed very near the speed of light ), and because particles of a particular energy have a higher speed the lighter they are, particle paths with many very strong signals can be identified as belonging to the lightest charged particles : electrons and their antiparticles, positrons.
