Dilithium is the only substance that can safely combine matter and anti-matter into a stable stream.

The Crystal becomes the focal point where the matter/anti-matter injectors fire. The crystals then combine and refract the beam in to the Warp Core. Kinda like the reverse of a light prism. Instead of only one beam of light hitting the prism and refracting into mutiple beams of light.. it does the exact opposite. Takes 2 beams of in-compatable energy, combines them to make a single stable beam of energy and fires into the warp core chamber... and voila, you have warp power!!

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Alshrim Dax