Dark matter made of neutrons would take more time to form a disk. Ballpark estimate: neutrons are 50e3 times smaller, but we need 5x and of them, so dark matter is 10e3 slower at forming disks.
In this model, neutrons aren't decaying: dark matter consists of "stable neutrons" that have a different quark structure, but are very similar to regular neutrons in all other aspects: no electric charge and a small magnetic moment.
I don't think we need to explain everything before testing this hypothesis. Launching a sat to the supposed dark halo responsible for the flyby anomaly seems a relatively trivial task these days.
It seems you want to kill two birds with one stone: flyby anomaly is caused by dark matter, which is actually "stable neutrons".
The problem is that "stable neutrons" seems to be a unicorn, not a stone.
After all, if a "stable neutron" is just somehow the same quarks in a different configuration, then how come it has eluded detector experiments ala LHC and friends?
Especially in light of the neutron freeze-out in the early universe? That is, how come the early universe ended up with way more "stable neutrons" than regular neutrons, yet somehow our experiments which regularly make neutrons has not noticed them? The missing mass should be very obvious.
You should at the very least have a plausible answer for that before spending the considerable amount of money and effort it takes to make and launch a probe.
That stable neutron can be a stable tetraquark with zero electric charge, or something of that sort.
It has been eluding lhc experiments for the same reason dark matter has been eluding them: detecting neutron like particles is extremely difficult when there are only 5 of them per cm3. I suppose that in some experiments those neutral particles get hit by accident and physicists see some discrepancies in the results, but since those discrepancies aren't reproducible, they get swiped under the rug.
As for the early universe model, well, it's a nice theory and it's backed by a few indirect evidences, but it's still just a theory we can't verify directly. This theory invented before the dark matter came to light, right?
The experiment we're talking about would cost maybe 10 billions - a rounding error in a world where some people make multiples of that in just a few months.
In this model, neutrons aren't decaying: dark matter consists of "stable neutrons" that have a different quark structure, but are very similar to regular neutrons in all other aspects: no electric charge and a small magnetic moment.
I don't think we need to explain everything before testing this hypothesis. Launching a sat to the supposed dark halo responsible for the flyby anomaly seems a relatively trivial task these days.