If your "eyes" could only "see" gamma rays, and yet you somehow managed to reach a level of science that was at least where we are, you would be theorizing/calculating that some other form of energy (like we see in the different wavelenghts of "light") must exist, and you would probably refer to that missing energy/matter as "dark".
I think calling an element of the universe we simply do not have the eyes or the technology to directly detect as "dark" gives a negative, pejorative and subjective value to something that might be real but lie just beyond our present level of ignorance.
It is actually our level of understanding that is still "dark" and not some aspect of the universe we are not yet smart enough to "see".
It's not pejorative. It's literally true. Whatever it is, it does not couple to electromagnetism; it cannot radiate or absorb light. It's not just a matter of a higher or lower frequency. The only handle we have on it (indeed, the only one we can have) is gravity. That's how it was first inferred, and that's how this map was made.
bttt