Dark matter actually fit very well. The chances it is there are extremely strong when you consider galaxy rotation curves, gravitational lensing data, cosmic microwave background analyses, etc... The only missing ingredient here is its direct detection. But it is not because we are not detecting it that it is not there. Our experiments have some sensitivity, and there are not built to cover all possible scenarios (hopefully they will in a couple of decades).

I have one question however, and that is why do we think there is any relationship between dark matter baryon's and neutron decay in the first place?

It is one option as valid as others. What is good is to be sure to consider everything and not leaver any loophole in our exploration.

I hope this clarifies!