Thanks for your message. There are several questions in there. Let's first answer the first one:

So, as of now it is unclear that what happens to these decayed neutrons

This is indeed a hot topic of research. We don't know with enough certainty what could be the cause of the difference between the two measurements, and we are trying to explore all options. The dark neutron decay is one of them.

Also I want to know whether the existence of Dark Matter still a theoretical concept?

There is a lot of experimental evidence pointing to the existence of dark matter. However, for some reasons, dark matter still evades direct detection and we have thus very little information on its nature. As above, we are trying to explore all options.

Note that theories without dark matter, in which gravity is modified, are also viable alternative. But they are not as strongly supported by data (even if not excluded).