This is partially true. Dark matter is hypothetical, but if dark matter is not there, you need something else. And this something else is not something known (at is part of our universe).

Secondly, your definition of a cosmic ray is wrong. Cosmic rays can be neutrinos, photons, protons, electrons, etc. Wikipedia is sometimes very inaccurate... ;)
And cosmic rays exist. They are observed and their composition is known.