Can anybody here describe the likely fate of a free neutron whizzing across the universe? What stops it or makes its combination into a nucleus somewhere more or less likely?

A free neutron has a half life of about 15 minutes, and will decay to form a proton, an electron and an antineutrino. The faster a neutron is travelling, the less likely it is to be absorbed into a nucleus. Elements with relatively small numbers of neutrons are good at absorbing them, for example boron or, I believe, cadmium. Slowing down a neutron just involves putting material in its way; I believe graphite was used in the first nuclear reactor.

thanks geodesic

once you told me that a neutron decays, I went looking and found a good reference (http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html#c3) to this. Now, I guess, the next question is, what happens to that proton? Pair up with an electron and make atomic hydrogen, then fall into a gravity well eventually?

A free neutron has a half life of about 15 minutesA free proton though lasts a spot longer ;)