I've never really explained this to anyone before...

[ But she supposed she had no choice. Nasu removed the small black, handle-like item from her pocket and showed it briefly to Asuka, then activated it. A flurry of light particles seemed to radiate from her form as a white Border agent uniform seemed to transposed onto her in place of her old clothing. A small, glowing green cube of light appeared just above her palm. ]

This cube is made of Trion. It's a type of energy, which all human beings naturally produce.