Most of the universe is believed to be
a. dark energy
b. dark matter
c. baryonic material

B. dark matter

Explanation:

The current theory is that the dark matter composes about 85% of the matter in the whole universe, possibly in large majority made out of unknown material.  What we found so far is that the majority of the dark matter is NOT composed of baryonic material (the stuff that makes stars and planet).

We know dark matter exists and we suppose there's a HUGE amount of it that fills the gaps between stars and planets.  

scientific laws are the result of testing theories and proving them to be facts.