That's not how it works.
Here we have a sterile neutrino population (definitely not interacting via electromagnetism and apparently not via weak force either) generally just sitting there like a heavy gas around galaxy cluster, but now slowly (i.e. with half-life times >> current age of the universe) decaying into a photon (the detected signal) and a standard neutrino (undetectable again).
It's only the decay signal you can observe. More clearly:
How can we see if this is the right model? Dark matter that has no interactions with the visible matter seems hopeless. Fortunately, sterile neutrino dark matter is expected to decay and produce a smoking-gun signal in the form of a monochromatic photon line. This is because, in order to be produced in the early universe, the sterile neutrino should mix slightly with the active ones. In that case, oscillations of the active neutrinos into sterile ones in the primordial plasma can populate the number density of sterile neutrinos, and by this mechanism it is possible to explain the observed relic density of dark matter. But the same mixing will make the sterile neutrino decay, as shown in the diagrams here. If the sterile neutrino is light enough and/or the mixing is small enough then its lifetime can be much longer than the age of the universe, and then it remains a viable dark matter candidate. The tree-level decay into 3 ordinary neutrinos is undetectable, but the 2-body loop decay into a photon and and a neutrino results in production of photons with the energy E=mDM/2. Such a monochromatic photon line can potentially be observed. In fact, in the simplest models sterile neutrino dark matter heavier than ~50 keV would produce a too large photon flux and is excluded. Thus the favored mass range for dark matter is between 1 and 50 keV. Then the photon line is predicted to fall into the X-ray domain that can be studied using X-ray satellites like XMM-Newton, Chandra, or Suzaku.
Also note that dark matter detectors assume that dark matter should interact at least via the weak force (they are "Weakly Interacting Massive Particles"). Same with the neutrino detectors. The electromagnetic force does not enter here.