It is a well-known fact that the universe is expanding. This means that the farther we look in the universe, the farther speed away the objects there are from us.
In this case, based on the Doppler Effect, the frequency of electromagnetic waves will be slide in a lower region.
This means that when you look at to a certain distance with the optical telescope, the colors shift, because of the Doppler Effect. Red going to be yellow, yellow will be green, etc., and magenta slides to UV (Ultra-violet).
It is not a major problem. However the visual will be false.
On the other hand, if we look at a much more distant region, the optical light could slide to UV region totally. If the distance is even greater, the optical light could slide into X-ray region, or even into microwaves. It is depend of distance between us and the object because due to the increasing distance, the speed of the object also greater, (compared to us) due to the expansion of universe.
In fact, the result is that optical light from this region will be invisible when viewed through an optical telescope.
Probably, this is the dark matter. It is there, but we not able to see it, through the optical telescope, because it shifted into the invisible region of electromagnetic waves, what is invisible for our eyes.
(If anything visible in this region through the optical telescope, it is the infrared or radio wave. That of one has slipped into the visible light range, because of the Doppler Effect.)