We have good reason to believe that the missing mass cannot be baryonic or fast moving massive particles. This comes from the abundance of elements, so it is in

We have good reason to believe that the missing mass cannot be baryonic or fast moving massive particles. This comes from the abundance of elements, so it is indirect but is consistent with other findings. Furthermore, those cosmic rays would radiate when decelerated. I think if there was substantial mass in cosmic rays, we would see this radiation (did not make a calculation though). Note that the estimate for the mass of dark matter is about ~5 times the normal matter.