Scripture: All spirit is matter, but it is more fine or pure, and can only be discerned by purer eyes.
Doctrine and Covenants 131:7
Science: Dark matter is matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly.If spirits are "matter," then God gets that matter from somewhere when he creates spirits. No problem, the latest estimates (from the same Wikipedia article) are that only 4% of the total energy density of the universe is visible matter (stars, planets, human bodies, etc.) That leaves 96% left over to create spirits from.
Wikipedia: Dark Matter
No idea if there's really any truth to this; "spirit matter" could be an entirely different type of matter that doesn't even have gravitational effects, much less electromagnetic ones. Still, it's an interesting idea.